Thoroughly enjoy each place when climbing (3,2)
I believe the answer is:
eat up
'thoroughly enjoy' is the definition.
(I've seen this in another clue)
'each place when climbing' is the wordplay.
'each' becomes 'ea' (abbreviation).
'place' becomes 'put' (synonyms).
'when climbing' says the letters should be written backwards (in a down clue, letters go up).
'put' in reverse letter order is 'tup'.
'ea'+'tup'='EAT UP'
(Other definitions for eat up that I've seen before include "Finish one's food" , "Exhaust" , "finish meal" , "Devour entirely" , "43 [TUCK] in" .)