Very fit, but distracted (5)
I believe the answer is:
vague
'distracted' is the definition.
'very fit' is the wordplay.
'very' becomes 'v' (abbreviation).
'fit' becomes 'ague' (I've seen this before).
'v'+'ague'='VAGUE'
'but' acts as a link.
(Other definitions for vague that I've seen before include "Indefinite" , "Not clearly defined or perceived" , "Unclear" , "Amorphous" , "Indistinct or imprecise" .)