Small, small net captures unknown eccentric barber (7,4)
I believe the answer is:
sweeney todd
'barber' is the definition.
(fictional barber and serial killer)
'small small net captures unknown eccentric' is the wordplay.
I cannot really see how this works, but
'small' could be 'wee' (similar in meaning) and 'wee' is found in the answer.
'unknown' could be 'y' (algebra) and 'y' is located in the answer.
'eccentric' could be 'odd' (I've seen this before) and 'odd' is found in the answer.
This explanation may well be incorrect...
(Other definitions for sweeney todd that I've seen before include "Serial killer" , "Demon barber of Fleet Street" , "Victorian villain" , "Murdering barber" , "used to refer slangily to police unit" .)