Girl training in gym originally getting fit (8)
I believe the answer is:
adapting
'getting fit' is the definition.
(making something fit for purpose)
'girl training in gym originally' is the wordplay.
'girl' becomes 'ada' (girl's name).
'training' becomes 'PT' (Physical Training).
'originally' indicates taking the first letters.
The initial letter of 'gym' is 'g'.
'ada'+'pt'+'in'+'g'='ADAPTING'
(Other definitions for adapting that I've seen before include "Adjusting to suit a new use" , "Changing to suit a new purpose" , "Changing to fit new circumstances" , "Altering to fit another purpose" .)