Cattle releasing core part of gas (4)
I believe the answer is:
oxen
'cattle' is the definition.
('oxen' can be a synonym of 'cattle')
'releasing core part of gas' is the wordplay.
'releasing core part of' suggests removing the centre (the middle or core is removed).
'gas' becomes 'oxygen' (I've seen this before).
'oxygen' with its middle taken out is 'OXEN'.
(Other definitions for oxen that I've seen before include "Bovine animals, in yoke" , "Horned farm animals" , "strong animals" , "Draught animals" , "Bovine mammals" .)