In favour of class losing marks (3)
I believe the answer is:
for
'in favour of' is the definition.
(I've seen this before)
'class losing marks' is the wordplay.
'class' becomes 'form' (form class is a kind of class).
'losing' is a deletion indicator.
'marks' becomes 'm' (former German currency).
'form' with 'm' removed is 'FOR'.
(Other definitions for for that I've seen before include "On the side of" , "Favouring" , "and 16 and 20: Notwithstanding" , "Pro" , "Approving" .)