Building to the west end of Westminster (5)
I believe the answer is:
tower
'building' is the definition.
(I've seen this before)
'the west end of westminster' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'west end of westminster' becomes 'ower' (I can't justify this - if you can you should give a lot more credence to this answer).
't'+'ower'='TOWER'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for tower that I've seen before include "Pisa has a leaning one" , "One drawing" , "Tall feature of Blackpool" , "famous prison" , "Perhaps keep" .)