Queue seen around a vacant two-seater toilet (7)
I believe the answer is:
latrine
'toilet' is the definition.
(latrine is a kind of toilet)
'queue seen around a vacant two-seater' is the wordplay.
'queue' becomes 'line' (a line of people).
'seen around' is an insertion indicator.
'vacant two seater' becomes 'tr' (I can't explain this - if you can you should believe this answer much more).
'a'+'tr'='atr'
'line' placed around 'atr' is 'LATRINE'.
Can you help me to learn more?
(Other definitions for latrine that I've seen before include "waste disposal unit" , "Camp lavatory" , "Where one may go" , "Gentlemen's" , "Army lavatory" .)