Struggle to take exam again outside school (6)
I believe the answer is:
resist
'struggle' is the definition.
(I know that resist is a more specific form of the action defend)
'take exam again outside school' is the wordplay.
'take exam again' becomes 'resit' (sit an exam again).
'outside' is an insertion indicator.
'school' becomes 'S'.
'resit' placed around 's' is 'RESIST'.
'to' acts as a link.
(Other definitions for resist that I've seen before include "Don't give in" , "Oppose or fight against someone" , "Combat" , "Defy for sister" , "Sister (anag)" .)