Cold climb posing problem for athlete? (5)
I believe the answer is:
cramp
'posing problem for athlete?' is the definition.
(an athlete may get cramp)
'cold climb' is the wordplay.
'cold' becomes 'c' (eg on taps).
'climb' becomes 'ramp' (ramping is a kind of climbing).
'c'+'ramp'='CRAMP'
(Other definitions for cramp that I've seen before include "Involuntary muscular contraction" , "To restrict; a muscle pain" , "Painful muscle problem" , "Confine - muscular spasm" , "Involuntary and painful contraction of a muscle" .)