Also equally fit (2,4)
I believe the answer is:
as well
'also' is the definition.
('as well' can be a synonym of 'also')
'equally fit' is the wordplay.
'equally' becomes 'as' ('as' can be a synonym of 'equally').
'fit' becomes 'well' ('well' is associated in meaning with 'fit').
'as'+'well'='AS WELL'
(Other definitions for as well that I've seen before include "A swell sort of addition" , "with equal effect" , "Also, in addition" , "on top" , "Also, additionally" .)