Group from London area (close to Barnet) (3)
I believe the answer is:
set
'group' is the definition.
(both can mean a collection)
'london area close to barnet' is the wordplay.
'london area' becomes 'SE' (South East postcode).
'close to' suggests the final letters (the close of something is the end).
The last letter of 'barnet' is 't'.
'se'+'t'='SET'
'from' acts as a link.
(Other definitions for set that I've seen before include "Fix in place; similar group" , "Group; hard" , "Create a puzzle" , "Gel; start (a fire)" , "Hardened" .)