Tin and nickel originally covering old gun (6)
I believe the answer is:
cannon
'gun' is the definition.
(I know that cannon is a type of gun)
'tin and nickel originally covering old' is the wordplay.
'tin' becomes 'can' (synonyms).
'and' becomes 'n' (common abbreviation for 'and').
'originally' indicates taking the first letters.
'covering' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
The initial letter of 'nickel' is 'n'.
'n'+'n'='nn'
'nn' going around 'o' is 'non'.
'can'+'non'='CANNON'
(Other definitions for cannon that I've seen before include "Field gun" , "Large artillery piece" , "Wheeled guns" , "military weapon" , "Big shot" .)