Manage firm exercise (4)
I believe the answer is:
cope
'manage' is the definition.
(manage can mean to cope or do well)
'firm exercise' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'exercise' becomes 'pe' (Physical Education).
'co'+'pe'='COPE'
(Other definitions for cope that I've seen before include "Long cloak for a priest" , "top on wall" , "Manage to deal with the vestment" , "investment" , "Manage, make do" .)