To stick together he must be in the centre (6)
I believe the answer is:
cohere
'to stick together' is the definition.
(to cohere is to stick or fit together)
'he must be in the centre' is the wordplay.
'must be in' indicates putting letters inside.
'the centre' becomes 'core' (core can mean something's centre).
'he' inserted within 'core' is 'COHERE'.
(Other definitions for cohere that I've seen before include "Fit together" , "Stick together, remain united" , "Be consistent" , "Fuse together" , "Hang together" .)