The building of a tug? (5)
I believe the answer is:
tower
I believe this is a double definition.
'the building' is the first definition.
(I've seen this before)
'a tug?' is the second definition.
(something which tugs or tows)
'of' is the link.
(Other definitions for tower that I've seen before include "See 4 Down" , "Wrote (anag.)" , "Pisa has a leaning one" , "Stand very high (over)" , "Rise to great height" .)