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