There's one in outer Hackney, darling (5)
I believe the answer is:
honey
'darling' is the definition.
(I've seen this in another clue)
'there's one in outer hackney' is the wordplay.
'there's' becomes 'one' (I can't justify this - if you can you should believe this answer much more).
'one in' means one lot of letters goes inside another.
'outer' suggests removing the centre.
'hackney' with its centre removed is 'hy'.
'one' put into 'hy' is 'HONEY'.
Can you help me to learn more?
(Other definitions for honey that I've seen before include "Sweet term of endearment" , "sweetener" , "Love" , "Hive fluid" , "Sweet stuff - darling" .)