Girl with exercises to modify (5)
I believe the answer is:
adapt
'to modify' is the definition.
(to modify is to improve or adapt)
'girl with exercises' is the wordplay.
'girl' becomes 'ada' (girl's name).
'with' means one lot of letters go next to another.
'exercises' becomes 'PT' (Physical Training).
'ada'+'pt'='ADAPT'
(Other definitions for adapt that I've seen before include "Fashion" , "Become adjusted to new conditions" , "Adjust (to changes)" , "Change to suit a different purpose" , "Modify; acclimatise" .)