Stick ambassador in the middle (6)
I believe the answer is:
cohere
'stick' is the definition.
('cohere' can be a synonym of 'stick')
'ambassador in the middle' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'in the' indicates putting letters inside.
'middle' becomes 'core'.
'he' inserted into 'core' is 'COHERE'.
(Other definitions for cohere that I've seen before include "Stick firmly together" , "Cling" , "Hold firmly together" , "Be consistent" , "Fit together" .)