Definitely not the very smallest (5,2,3)

'definitely' is the definition.
Both the definition and answer are adverbs. Perhaps you can see a link between them that I can't see?

'the very smallest' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'very' could be 'so' (both can mean 'extremely') and 'so' is found within the leftover letters.
This explanation may well be incorrect...

'not' acts as a link.

Can you help me to learn more?