Popular theatre left prop outside without hesitation (9)
I believe the answer is:
instantly
'without hesitation' is the definition.
Both the answer and definition are adverbs. Maybe you can see a link between them that I can't see?
'popular theatre left prop outside' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'theatre left' becomes 'ntl' (I can't justify this - if you can you should believe this answer much more).
'prop' becomes 'stay' (I've seen this in another clue).
'outside' indicates putting letters inside.
'ntl' going inside 'stay' is 'stantly'.
'in'+'stantly'='INSTANTLY'
Can you help me to learn more?
(Other definitions for instantly that I've seen before include "Without delay or hesitation" , "Immediately, forthwith" , "With no delay, this very second" .)