During escape, rescuer ascending fissure in glacier (8)
I believe the answer is:
crevasse
'fissure in glacier' is the definition.
(I've seen this before)
'during escape rescuer ascending' is the wordplay.
'during' indicates putting letters inside.
'escape' becomes 'esc' (Escape key on a keyboard).
'rescuer' becomes 'saver' (synonyms).
'ascending' is a reversal indicator.
'esc' placed around 'saver' is 'essaverc'.
'essaverc' back-to-front is 'CREVASSE'.
(Other definitions for crevasse that I've seen before include "Crack in a mountain slope or a glacier" , "cold break" , "13 in a glacier" , "Sever sac (anag)" , "climbing hazard in glacier" .)