He's in the middle, making us stick together (6)
I believe the answer is:
cohere
'stick together' is the definition.
(to cohere is to stick or fit together)
'he's in the middle' is the wordplay.
'in' means one lot of letters goes inside another.
'the middle' becomes 'core' (core can mean the centre of something).
'he' put inside 'core' is 'COHERE'.
'making us' acts as a link.
(Other definitions for cohere that I've seen before include "Even; without jerks" , "Fuse together" , "Hold firmly together" , "Stick firmly together" , "Stick or hold together and resist separation" .)