Lament hem being ruined by dairy product (9)
I believe the answer is:
emmenthal
'dairy product' is the definition.
(Emmenthal is an example)
'lament hem being ruined' is the wordplay.
'being ruined' indicates an anagram.
'lament'+'hem'='lamenthem'
'lamenthem' anagrammed gives 'EMMENTHAL'.
'by' acts as a link.
(Other definitions for emmenthal that I've seen before include "Swiss valley giving its name to a cheese" , "Hard Swiss cheese" , "Something cheesy" , "Type of cheese" , "Swiss cheese with holes" .)