Manages firm exercise with son (5)
I believe the answer is:
copes
'manages' is the definition.
('cope' can be a synonym of 'manage')
'firm exercise with son' is the wordplay.
'firm' becomes 'co' (abbreviation for company).
'exercise' becomes 'pe' (Physical Education).
'with' says to put letters next to each other.
'son' becomes 's' (genealogical abbreviation for son).
'co'+'pe'+'s'='COPES'
(Other definitions for copes that I've seen before include "Somehow manages" , "Manages successfully, gets by" , "'Manages, gets by (5)'" , "Manages; church vestments" .)