Capital tugboat is a landmark (5,2,6)
I believe the answer is:
tower of london
'a landmark' is the definition.
Both the definition and answer are singular nouns.
Maybe there's a link between them I don't understand?
'capital tugboat' is the wordplay.
I cannot really understand how this works, but
'capital' could be 'london' (capital of the United Kingdom) and 'london' is located in the answer.
'tugboat' could be 'tower' (I've seen this before) and 'tower' is present in the answer.
The remaining letters 'of' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'is' is the link.
Can you help me to learn more?
(Other definitions for tower of london that I've seen before include "Capital's castle" , "Where the Crown Jewels are kept" , "Building" , "Where to see crown jewels" , "Home to the UK's Crown jewels" .)