Old gun given to big cat (4,3)

'old gun' is the definition.
'long tom' can be an answer for 'gun' (long tom is a kind of gun). I am not certain of the 'old' bit.

'given to big cat' is the wordplay.
'given to' is a charade indicator (letters next to each other).
'big' becomes 'long' (I can't justify this - if you can you should give a lot more credence to this answer).
'cat' becomes 'tom' (a tom is a male cat).
'long'+'tom' is 'LONG TOM'.

Can you help me to learn more?