Months in laboratory producing meat (4)
I believe the answer is:
lamb
'meat' is the definition.
(lamb is a kind of meat)
'months in laboratory' is the wordplay.
'months' becomes 'm'.
'in' indicates putting letters inside.
'laboratory' becomes 'lab' (synonyms).
'm' placed into 'lab' is 'LAMB'.
'producing' acts as a link.
(Other definitions for lamb that I've seen before include "Meaty" , "Sweet and innocent child" , "Young farm animal" , "Young creature" , "Exemplar of innocence" .)