To be consistent he must be in the middle (6)
I believe the answer is:
'to be consistent' is the definition.
(I've seen this before)
'he must be in the middle' is the wordplay.
'must be in' is an insertion indicator.
'the middle' becomes 'core' (core can mean the centre of something).
'he' put into 'core' is 'COHERE'.
(Other definitions for cohere that I've seen before include "Cling" , "Be consistent" , "Form a whole" , "Stick or hold together and resist separation" , "Stick together, remain united" .)