Noisily chew meat cut by Monsieur (5)
I believe the answer is:
chomp
'noisily chew' is the definition.
(I know that to chomp is to chew noisily)
'meat cut by monsieur' is the wordplay.
'meat' becomes 'chop' (chop is a kind of meat).
'cut by' is an insertion indicator (some letters cut into others).
'monsieur' becomes 'm' (French abbreviation - like Mr for Mister).
'chop' placed around 'm' is 'CHOMP'.
(Other definitions for chomp that I've seen before include "Munch noisily (colloq.)" , "Chew" , "For crunch" .)