Jerseys, say, Conservative PM once shortened (6)
I believe the answer is:
cattle
'jerseys' is the definition.
(I've seen this in another clue)
'conservative pm once shortened' is the wordplay.
'conservative' becomes 'C' (UK political abbreviation).
'pm once' becomes 'Attlee' (prime minister Clement Attlee).
'shortened' means to remove the last letter.
'attlee' with its last letter taken away is 'attle'.
'c'+'attle'='CATTLE'
'say' acts as a link.
(Other definitions for cattle that I've seen before include "Domestic bovines" , "Farm livestock" , "feature of 21/23 [NATIVITY SCENE]" , "Lowers" , "Animals bred for milk or meat" .)