Tower in which may be seen some unhappy Londoners (5)
I believe the answer is:
pylon
'tower in which may be' is the definition.
'pylon' can be an answer for 'tower' (tower-like structure carrying electrical cables). I am not certain of the remainder of the definition.
'some unhappy londoners' is the wordplay.
'some' says the answer is hidden in the clue (some parts of the word are used).
'PYLON' is hidden in the letters of 'unhappy londoners'.
'seen' acts as a link.
Can you help me to learn more?
(Other definitions for pylon that I've seen before include "Steel tower supporting high-tension wires" , "Grid cable support" , "Power cable supporter" , "power supplier?" , "Tower for carrying power line" .)