The middle (6)
I believe the answer is:
centre
'the middle' is the definition.
('centre' can be a synonym of 'middle')
This is the entire clue.
(Other definitions for centre that I've seen before include "Heart" , "'Things fall apart; the ... cannot hold' (Yeats)" , "and this may follow 26, 25 [LEFT RIGHT]" , "See 7" , "Point equidistant from all other points" .)