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' is an anagram indicator.
'grade' anagrammed gives 'RAGED'.
'at' acts as a link.
(Other definitions for raged that I've seen before include "Expressed intense anger" , "was having fits" , "seethed" , "Behaved violently" , "Felt intense anger" .)