Hold firm’s ambassador in centre (6)
I believe the answer is:
cohere
'hold firm's' is the definition.
(to cohere is to hold firm or be consistent)
'ambassador in centre' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'in' means one lot of letters goes inside another.
'centre' becomes 'core' (core can mean something's centre).
'he' put inside 'core' is 'COHERE'.
(Other definitions for cohere that I've seen before include "Even; without jerks" , "Hold firmly together" , "Fuse together" , "Hang together" , "Fit together" .)