Stick gas in middle (6)
I believe the answer is:
cohere
'stick' is the definition.
(I know that stick can be written as cohere)
'gas in middle' is the wordplay.
'gas' becomes 'He' (chemical symbol for helium).
'in' means one lot of letters goes inside another.
'middle' becomes 'core' (core can mean the centre of something).
'he' put inside 'core' is 'COHERE'.
(Other definitions for cohere that I've seen before include "Form a whole" , "Stick firmly together" , "Fuse together" , "Be consistent" , "Hold firmly together" .)