Key exercise to cut pound? (6)
I believe the answer is:
opener
'key' is the definition.
(thesaurus)
'exercise to cut pound?' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'to cut' indicates putting letters inside.
'pound?' becomes 'oner' (informal term for one pound).
'pe' going within 'oner' is 'OPENER'.
(Other definitions for opener that I've seen before include "First act" , "First part" , "No.1 or No.2 bat" , "A key or a bottle top remover" , "introduction" .)