Lamb’s progenitor is core to sacrament (3)
I believe the answer is:
ram
'lamb's progenitor' is the definition.
(male parent or progenitor of a lamb)
'core to sacrament' is the wordplay.
'core to' says the answer is hidden in the clue (the answer is hidden at the core of the clue).
'RAM' is hidden in the letters of 'sacrament'.
'is' is the link.
(Other definitions for ram that I've seen before include "Prepare to raid" , "what comes after fish" , "Butter" , "Object to batter with" , "Bump into forcibly" .)