Getting on with borders to trim, having consumed zero gin (3,3)

I believe the answer is:

old tom

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'trim having consumed zero gin' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'getting on with borders to' is the wordplay.
'getting on' becomes 'old' ('getting on a bit' means 'old').
'with' becomes 'm' (I can't explain this - if you can you should believe this answer much more).
'borders' means one lot of letters goes inside another.
'old'+'m'='oldm'
'oldm' enclosing 'to' is 'OLD TOM'.

Can you help me to learn more?

I've seen this clue in The Telegraph.

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