Into fitness, healthy woman (3)
I believe the answer is:
she
'woman' is the definition.
'into fitness healthy' is the wordplay.
'into' indicates the answer is hidden within the clue.
'SHE' is hidden within 'fitness healthy'.
(Other definitions for she that I've seen before include "way to refer to mare" , "Rider's novel" , "The one who must be obeyed" , "1887 Rider Haggard novel" , "H. Rider Haggard novel" .)