Trump not in form (8)
I believe the answer is:
outclass
'trump' is the definition.
(I've seen this before)
'not in form' is the wordplay.
'not in' becomes 'out' ('out' is the opposite to 'in').
'form' becomes 'class' (synonyms).
'out'+'class'='OUTCLASS'
(Other definitions for outclass that I've seen before include "Win easily" , "Thrash" , "Trump" , "put in the shade" , "Top" .)