THERE'S NO SECRET about Deborah Shaffer's politics in her documentary, "Fire From the Mountain." She doesn't want the Reagan administration (or any other administration) in Nicaragua. And, retracing the odyssey of Sandinista guerrilla Omar Cabezas' struggle against the Somoza regime (and now the United States), she makes a spirited effort to show Nicaraguan life as it was and is. In one hour.

Shaffer, who made last year's Oscar- winning documentary "Witness to War: Dr. Charlie Clements," has much ground to cover. With well-edited stock footage, interviews with pro-Sandinista people and readings from Cabezas' autobiographical book "Fire From the Mountain," she sketches the events before and after the 1979 insurrection. But Shaffer's film can't possibly do justice to Cabezas' impassioned and poetic account of his lengthy guerrilla training in the mountains in the late 1970s.

Shaffer takes the camera to scenes from Cabezas' past (the sun-baked streets, the jam-packed pool hall of his hometown) and lets Cabezas' spoken words (dubbed in English) do the rest. A born raconteur, Cabezas talks of the guerrilla myth as the "spark" that started the revolution, leading to insurrection in the cities and "every day, more and more people in the streets . . . losing their fear." Students would make brazier fires on street corners to attract a crowd. And when these gatherings led to discussions, fires suddenly "gained a subversive character . . . It was a people in flames that later became a people in arms."

The wit comes through when Cabezas tells of arriving at the jungle camp. He had given up a university degree, trekked through forested mountains, and expected to find an army. Instead there were barely 20 people. " 'Son of a bitch,' you say to yourself . . . 'My God, I made the worst decision of my life.' ". And when the Sandinistas had overrun the National Palace, says Cabezas, "I don't remember which companero said, 'Well, now what do we do?' "

Some will object to Shaffer's lack of journalistic balance, but "Fire" bears watching not so much for its politics as for its close look at the struggles of a people at war.

FIRE FROM THE MOUNTAIN (Unrated) --

At the Biograph.