Exertion makes us alter our belt (7)
I believe the answer is:
trouble
'exertion' is the definition.
(I know that trouble is a type of exertion)
'alter our belt' is the wordplay.
'alter' is an anagram indicator.
'our'+'belt'='ourbelt'
'ourbelt' with letters rearranged gives 'TROUBLE'.
'makes us' acts as a link.
(Other definitions for trouble that I've seen before include "Disquiet" , "Misfortune" , "Conflict" , "'A... shared is a... halved'" , "Disturb; unpleasantness" .)