Some unhappy Londoners in the Tower (5)
I believe the answer is:
pylon
'the tower' is the definition.
(pylon is a kind of tower)
'some unhappy londoners in' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'PYLON' is hidden within 'unhappy londoners in'.
(Other definitions for pylon that I've seen before include "Steel tower supporting high-tension wires" , "Tower or tall post" , "Electric grid support" , "Bearer of cable" , "Electric cable support" .)