Seethed at bad grade (5)
I believe the answer is:
raged
'seethed' is the definition.
(I've seen this before)
'bad grade' is the wordplay.
'bad' indicates an anagram.
'grade' with letters rearranged gives 'RAGED'.
'at' is the link.
(Other definitions for raged that I've seen before include "Expressed intense anger" , "was having fits" , "seethed" , "Behaved violently" , "Felt intense anger" .)