Third guy like the Gallagher brothers? (4)

I believe the answer is:
manc
'the gallagher brothers?' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'third guy' is the wordplay.
I cannot really understand how this works, but
'guy' could be 'man' (guy can informally mean a man) and 'man' is found in the answer.
A single letter 'c' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'like' acts as a link.
Can you help me to learn more?
I've seen this clue in The Independent.
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