Intense climbing in rock sport (8)
I believe the answer is:
speedway
'sport' is the definition.
(I've seen this before)
'intense climbing in rock' is the wordplay.
'intense' becomes 'deep' (similar in meaning).
'climbing' is a reversal indicator (in a down clue, letters climb up).
'in' indicates putting letters inside.
'rock' becomes 'sway' (synonyms).
'deep' in reverse letter order is 'peed'.
'peed' inserted within 'sway' is 'SPEEDWAY'.
(Other definitions for speedway that I've seen before include "Race-track" , "Cycling track" , "Stadium for motorcycle racing" , "Dirt track motorcycle racing" , "fast-track competition" .)