One possibly on vacation visiting London landmark, a riveting place (8)
I believe the answer is:
shipyard
'a riveting place' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
'one possibly on vacation visiting london landmark' is the wordplay.
'one' becomes 'i' (Roman numeral).
'on vacation' means to remove the middle letters (after the inside letters have vacated the word).
'visiting' means one lot of letters goes inside another.
'london landmark' becomes 'shard' (I can't explain this - if you can you should give a lot more credence to this answer).
'possibly' with its centre removed is 'py'.
'i'+'py'='ipy'
'ipy' inserted within 'shard' is 'SHIPYARD'.
Can you help me to learn more?
(Other definitions for shipyard that I've seen before include "Marine constructors" , "Area where large boats built" , "Place of marine construction and repair" , "Place where vessels are built" , "What might have a slipway" .)