On form, maybe; nevertheless rested (3)
I believe the answer is:
sat
'rested' is the definition.
(to sit is to rest)
'on form maybe nevertheless' is the wordplay.
I cannot really see how this works, but
'on' could be 'at' (I've seen this in other clues) and 'at' is found in the answer.
A single letter 's' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for sat that I've seen before include "Took (examination)" , "Rested" , "Pulled up a 29 down" , "Was sitting" , "Posed (for an artist)" .)