Before the rest centre (3)
I believe the answer is:
ere
'before' is the definition.
(poetic term for 'before')
'the rest centre' is the wordplay.
'centre' indicates the answer is hidden within the clue (hidden in the middle of the letters).
'ERE' is hidden within 'the rest'.
(Other definitions for ere that I've seen before include "Old prior" , "Before (poetic)" , "Before (in time)" , "Old word for before" , "Before, poetically" .)