The guys inside finished and perfected the whole (12)

'perfected the whole' is the definition.
(as in a part complementing a whole)

'the guys inside finished' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
'guys' could be 'men' (guy can mean a man) and 'men' is found within the answer.
This explanation may well be incorrect...

'and' is the link.

Can you help me to learn more?