Gloomy inside leaky marquee (10)
I believe the answer is:
portentous
'gloomy' is the definition.
(thesaurus)
'inside leaky marquee' is the wordplay.
'inside' means one lot of letters goes inside another.
'leaky' becomes 'porous' ('porous' can be similar in meaning to 'leaky').
'marquee' becomes 'tent' (I've seen this before).
'porous' going around 'tent' is 'PORTENTOUS'.
(Other definitions for portentous that I've seen before include "Of ominous meaning" , "Of great significance" .)