Todd Small, very small, by the sound of it (7)
I believe the answer is:
sweeney
'sound of it' is the definition.
The definition suggests a singular noun which matches the answer.
'todd small very small by' is the wordplay.
'todd' becomes 'ney' (I can't explain this - if you can you should give a lot more credence to this answer).
'small' becomes 's' (abbreviation - e.g. clothes size).
'very small' becomes 'wee' ('wee' is a Scottish word for small).
'by' says to put letters next to each other.
's'+'wee'='swee'
'ney' after 'swee' is 'SWEENEY'.
'the' is the link.
Can you help me to learn more?
(Other definitions for sweeney that I've seen before include "------- Todd, the 'demon barber of Fleet Street'" , "The flying squad" , "London police" .)