Stick high explosive in nucleus (6)
I believe the answer is:
cohere
'stick' is the definition.
('cohere' can be a synonym of 'stick')
'high explosive in nucleus' is the wordplay.
'high explosive' becomes 'he'.
'in' indicates putting letters inside.
'nucleus' becomes 'core' ('core' can be a synonym of 'nucleus').
'he' put into 'core' is 'COHERE'.
(Other definitions for cohere that I've seen before include "Hold firmly together" , "Fuse together" , "Be consistent" , "Stick firmly together" , "Stick together, remain united" .)