Make something inside at the opening (6)
I believe the answer is:
invent
'make' is the definition.
(I know that invent is a more specific form of the action concoct)
'inside at the opening' is the wordplay.
'inside' becomes 'in'.
'at' means one lot of letters go next to another.
'the opening' becomes 'vent' (vent is a kind of opening).
'in'+'vent'='INVENT'
'something' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for invent that I've seen before include "Make or design something original" , "Discover Devise, concoct" , "Design - discover" , "Create like Edison" , "Think up, create" .)