Rennet's first to be accepted by man before making cheese (7)
I believe the answer is:
gruyere
'cheese' is the definition.
(Gruyere is an example)
'rennet's first to be accepted by man before' is the wordplay.
'first' indicates taking the first letters.
'to be accepted by' is an insertion indicator (inserted letters are accepted).
'man' becomes 'guy' (guy is a kind of man).
'before' becomes 'ere' (poetic term).
The first letter of 'rennet' is 'r'.
'r' put within 'guy' is 'gruy'.
'gruy'+'ere'='GRUYERE'
'making' is the link.
(Other definitions for gruyere that I've seen before include "Whole-milk holey cheese" , "Firm Swiss cheese" , "Swiss cheese with small holes" , "Firm pale cheese with holes" , "Tangy cheese" .)