Say yes to a climb (6)
I believe the answer is:
ascent
'to a climb' is the definition.
(synonyms)
'say yes' is the wordplay.
'say' indicates a 'sounds like' (homophone) clue.
'yes' becomes 'assent' (I've seen this in another clue).
'assent' sounds like 'ASCENT'.
(Other definitions for ascent that I've seen before include "Upward slope" , "Going up" , "Act of moving upwards" , "Upward movement" , "Upward climb" .)