Cattle, a sort to produce milk (7)
I believe the answer is:
lactate
'produce milk' is the definition.
(I've seen this before)
'cattle a sort' is the wordplay.
'sort' indicates anagramming the letters.
'cattle'+'a'='cattlea'
'cattlea' with letters rearranged gives 'LACTATE'.
'to' is the link.
(Other definitions for lactate that I've seen before include "Exude milk" , "Produce milk" , "(Of female mammal) secrete milk" .)