Potter needs it, prompt! (3)
I believe the answer is:
cue
This clue is a double definition.
'potter needs it' is the first definition.
(a cue is used to pot balls in snooker)
'prompt' is the second definition.
(synonyms)
(Other definitions for cue that I've seen before include "Potter" , "Hint, prompt" , "Signal to begin - tapered stick" , "Catchword" , "Prompt; signal" .)