Girl having exercises to modify (5)
I believe the answer is:
adapt
'to modify' is the definition.
(to modify is to improve or adapt)
'girl having exercises' is the wordplay.
'girl' becomes 'ada' (girl's name).
'having' is a charade indicator (letters next to each other).
'exercises' becomes 'PT' (Physical Training).
'ada'+'pt'='ADAPT'
(Other definitions for adapt that I've seen before include "Become adjusted to new conditions" , "Fashion" , "Acclimatize" , "Put to particular use" , "Alter, amend" .)