Get the better of when not in form (8)
I believe the answer is:
outclass
'get the better of' is the definition.
(thesaurus)
'not in form' is the wordplay.
'not in' becomes 'out' ('out' is the opposite to 'in').
'form' becomes 'class' ('class' can be a synonym of 'form'**).
'out'+'class'='OUTCLASS'
'when' acts as a link.
(Other definitions for outclass that I've seen before include "Do much better than" , "Surpass greatly, be in a superior category" , "put in the shade" , "Trump" , "Be superior to (another)" .)