Offspring must eat lean dairy product (7)
I believe the answer is:
stilton
'dairy product' is the definition.
(Stilton is an example)
'offspring must eat lean' is the wordplay.
'offspring' becomes 'son' (male offspring).
'must eat' is an insertion indicator.
'lean' becomes 'tilt' (synonyms).
'son' placed around 'tilt' is 'STILTON'.
(Other definitions for stilton that I've seen before include "Strong English cheese, originally from Leicestershire" , "Strong Eng. cheese" , "Rich, blue-veined cheese" , "Cheese named after Cambridgeshire town" , "Blue 4 across" .)