There's one in outer Hackney, darling (5)
I believe the answer is:
honey
'darling' is the definition.
(I've seen this before)
'there's one in outer hackney' is the wordplay.
'there's' becomes 'one' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one in' means one lot of letters goes inside another.
'outer' suggests removing the centre.
'hackney' with its middle taken out is 'hy'.
'one' going 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" .)