Prompt, having left a hint (4)
I believe the answer is:
clue
'a hint' is the definition.
(I know this)
'prompt having left' is the wordplay.
'prompt' becomes 'cue' (synonyms).
'having' is an insertion indicator.
'left' becomes 'L' (common abbreviation).
'cue' going around 'l' is 'CLUE'.
(Other definitions for clue that I've seen before include "one of 28 here" , "Crossword item" , "Luce (anag.)" , "For example, this" , "Helpful hint" .)