Spoil most of sovereign's tent (7)
I believe the answer is:
marquee
'tent' is the definition.
(I know that marquee is a type of tent)
'spoil most of sovereign's' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'most of' means to remove the last letter.
'sovereign' becomes 'queen' (I've seen this before).
'queen' with its last letter taken off is 'quee'.
'mar'+'quee'='MARQUEE'
(Other definitions for marquee that I've seen before include "Large and often sumptuous tent" , "Elaborate tent" , "Large tent used for party, exhibition etc" , "covering for hotel visitors there" , "event venue" .)