A broken bit in tall building (5)
I believe the answer is:
shard
I believe this clue is a double definition.
'a broken bit' is the first definition.
(broken bit of glass)
'tall building' is the second definition.
(London skyscraper)
'in' acts as a link.
(Other definitions for shard that I've seen before include "Sliver of glass, etc" , "construction in London" , "Sharp glass fragment" , "archaeological find" , "remnant after break-up" .)