An arm or leg? It’s familiar (4,2,3,9)

'familiar' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'an arm or leg? it's' is the wordplay.
I cannot quite understand how this works, but
'an' could be 'i' (Roman numeral for one) and 'i' is present in the answer.
'leg?' could be 'part' (leg is a kind of part) and 'part' is located in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?