Unhappy after climb cut back (5,4)
I believe the answer is:
scale down
'cut back' is the definition.
(thesaurus)
'unhappy after climb' is the wordplay.
'unhappy' becomes 'down' (I've seen this before).
'after' says to put letters next to each other.
'climb' becomes 'scale' (scaling is a kind of climbing).
'down' after 'scale' is 'SCALE DOWN'.
(Other definitions for scale down that I've seen before include "cut back" , "Reduce in proportion to original" , "Make smaller in proportion to original" .)