Concert part for theatre assistant (6)

I believe the answer is:
prompt
'theatre assistant' is the definition.
Both the definition and answer are adjectives. Maybe you can see a link between them that I don't see?
'concert part' is the wordplay.
'concert' becomes 'prom' (abbreviation for promenade concert).
'part' becomes 'pt' (abbreviation).
'prom'+'pt'='PROMPT'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for prompt that I've seen before include "Give cue to forgetful actor" , "Speedy - help for forgetful actor" , "Give hint as to actor who has forgotten lines (6)" , "Encourage" , "With little or no delay" .)
