Good cut of meat is beef (5)
I believe the answer is:
grump
'is beef' is the definition.
(I've seen this before)
'good cut of meat' is the wordplay.
'good' becomes 'g' (abbreviation).
'cut of meat' becomes 'rump' (rump is a kind of cut of meat).
'g'+'rump'='GRUMP'
(Other definitions for grump that I've seen before include "Sulky person" , "Sulky type" , "Surly person" , "pet?" , "Bad-tempered person" .)