Bind helium in nucleus (6)
I believe the answer is:
cohere
'bind' is the definition.
'helium in nucleus' is the wordplay.
'helium' becomes 'he' ('He' can be a synonym of 'helium').
'in' is an insertion indicator.
'nucleus' becomes 'core' ('core' can be a synonym of 'nucleus').
'he' placed within 'core' is 'COHERE'.
(Other definitions for cohere that I've seen before include "Fuse together" , "Form a whole" , "Stick together, remain united" , "Stick or hold together and resist separation" , "Stick firmly together" .)