Rust in the centre of the gun (7)
I believe the answer is:
corrode
'rust' is the definition.
(synonyms)
'in the centre of the gun' is the wordplay.
'in the' is an insertion indicator.
'centre of' becomes 'core' (I can't justify this - if you can you should believe this answer much more).
'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" .)