Stick cardinal in centre (6)
I believe the answer is:
cohere
'stick' is the definition.
('cohere' can be a synonym of 'stick')
'cardinal in centre' is the wordplay.
'cardinal' becomes 'he' (I've seen this in other clues).
'in' means one lot of letters goes inside another.
'centre' becomes 'core' (core is a kind of centre).
'he' placed within 'core' is 'COHERE'.
(Other definitions for cohere that I've seen before include "Fit together" , "Fuse together" , "Even; without jerks" , "Be consistent" , "Stick firmly together" .)