Bite of mustard's first met in some meat (5)
I believe the answer is:
chomp
'bite' is the definition.
(I know that bite can be written as chomp)
'mustard's first met in some meat' is the wordplay.
'mustard's first' becomes 'm' (cst letter of 'mustard').
'met in' is an insertion indicator.
'some meat' becomes 'chop' (chop is a kind of meat).
'm' inserted into 'chop' is 'CHOMP'.
'of' acts as a link.
(Other definitions for chomp that I've seen before include "Chew" , "For crunch" , "Munch noisily (colloq.)" .)