Fiend negotiated no terms (7)
I believe the answer is:
monster
'fiend' is the definition.
(I know that fiend can be written as monster)
'negotiated no terms' is the wordplay.
'negotiated' indicates anagramming the letters (negotiate can mean to organise or arrange).
'no'+'terms'='noterms'
'noterms' anagrammed gives 'MONSTER'.
(Other definitions for monster that I've seen before include "Strange mentors for the imaginary creature" , "something large and ugly" , "Large, mytical creature" , "Large and grotesque animal" , "Imaginary beast" .)