Lead actor’s reminder about line (4)
I believe the answer is:
clue
'lead' is the definition.
(I've seen this before)
'actor's reminder about line' is the wordplay.
'actor's reminder' becomes 'cue' (I've seen this in another clue).
'about' indicates putting letters inside.
'line' becomes 'l' (used when specifying particular lines from a poem).
'cue' enclosing 'l' is 'CLUE'.
(Other definitions for clue that I've seen before include "Indication, hint" , "For example, this" , "One of twenty-four in this puzzle" , "Luce (anag.)" , "Part of a crossword" .)