Rust in the centre of the gun (7)
I believe the answer is:
corrode
'rust' is the definition.
(I know that rust can be written as corrode)
'in the centre of the gun' is the wordplay.
'in the' is an insertion indicator.
'centre of' becomes 'core' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the gun' becomes 'rod' (rod is a kind of gun).
'core' placed around 'rod' is 'CORRODE'.
Can you help me to learn more?
(Other definitions for corrode that I've seen before include "Cause to rust" , "Chemically weaken" , "Slowly eat" , "Gradually eat away" , "Eat away, rust" .)