Stick helium in nuclear reactor (6)
I believe the answer is:
cohere
'stick' is the definition.
(I know that stick can be written as cohere)
'helium in nuclear reactor' is the wordplay.
'helium' becomes 'he' (chemical symbol).
'in' means one lot of letters goes inside another.
'nuclear reactor' becomes 'core' (I can't justify this - if you can you should believe this answer much more).
'he' going into 'core' is 'COHERE'.
Can you help me to learn more?
(Other definitions for cohere that I've seen before include "Fit together" , "Hold firmly together" , "Be consistent" , "Form a whole" , "Stick together, remain united" .)