Key rings for climbing can be stiff (6)
I believe the answer is:
corpse
'stiff' is the definition.
(synonyms)
'key rings for climbing' is the wordplay.
'key' becomes 'esc' ('Escape' key on a computer keyboard).
'rings' indicates putting letters inside (some letters are ringed or contained by others).
'for' becomes 'pro' (as in being 'pro' or in favour of something).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'esc' enclosing 'pro' is 'esproc'.
'esproc' backwards is 'CORPSE'.
'can be' acts as a link.
(Other definitions for corpse that I've seen before include "Dead bodies" , "Cadaver, dead person" , "Dead body" , "Roof beam" .)