Gift adult left in marquee (6)
I believe the answer is:
talent
'gift' is the definition.
('talent' can be a synonym of 'gift')
'adult left in marquee' is the wordplay.
'adult' becomes 'a'.
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'marquee' becomes 'tent' (I've seen this before).
'a'+'l'='al'
'al' going inside 'tent' is 'TALENT'.
(Other definitions for talent that I've seen before include "Innate ability or Biblical money unit" , "ancient coin" , "Special gift" , "Gift, flair" , "Innate skill" .)