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' means one lot of letters go next to another.
'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" .)