In addition to being equally fit (2,4,2)
I believe the answer is:
as well as
'in addition to' is the definition.
(I've seen this before)
'being equally fit' is the wordplay.
I cannot quite understand how this works, but
'being' could be 'swell' (swell is a kind of being) and 'swell' is located in the answer.
'equally' could be 'as' (synonyms) and 'as' is present in the answer.
A single letter 'a' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for as well as that I've seen before include "No less than" , "Supplementing" , "this can be expected when in reasonable health" , "In addition to (2, 4, 2)" , "Along with" .)