Check on form (8)
I believe the answer is:
'check' is the definition.
(I know that check is a kind of restrain)
'on form' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'form' becomes 'strain' (synonyms).
(Other definitions for restrain that I've seen before include "Keep in check" , "Manacle" , "Tie up - arrest" , "Keep under control" , "Hold back, tether" .)