Train is something essential for carrying sheep (7)
I believe the answer is:
cortege
'train' is the definition.
(both can mean a procession following behind)
'something essential for carrying sheep' is the wordplay.
'something essential' becomes 'core' (essential or central part of something).
'for carrying' indicates putting letters inside.
'sheep' becomes 'teg' (I've seen this before).
'core' enclosing 'teg' is 'CORTEGE'.
'is' is the link.
(Other definitions for cortege that I've seen before include "Entourage, retinue" , "Solemn funeral procession" , "Processing figures" , "train" .)