Stall supporting group that might be on the prom? (9)
I believe the answer is:
bandstand
'that might be on the prom?' is the definition.
(a bandstand might be located on a promenade)
'stall supporting group' is the wordplay.
'stall' becomes 'stand' (synonyms).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'group' becomes 'band' (band can mean a group of people).
'stand' put after 'band' is 'BANDSTAND'.
(Other definitions for bandstand that I've seen before include "place to play?" , "park feature?" , "Outdoor concert stage" , "Outdoor platform for musical performances etc" , "Park structure for musicians" .)