Very fit — also dim (5)
I believe the answer is:
vague
'dim' is the definition.
(I know that dim can be written as vague)
'very fit also' is the wordplay.
'very' becomes 'v' (abbreviation).
'fit' becomes 'ague' (I've seen this before).
'also' means one lot of letters go next to another.
'v'+'ague'='VAGUE'
(Other definitions for vague that I've seen before include "not focused" , "Hazy" , "Unclear" , "Not clearly defined or perceived" , "Indefinite" .)