Actor's signal about line — such as this (4)
I believe the answer is:
clue
'this' is the definition.
(the clue referring to itself!)
'actor's signal about line' is the wordplay.
'actor's signal' becomes 'cue' (I've seen this in another clue).
'about' is an insertion indicator.
'line' becomes 'l' (used when specifying particular lines from a poem).
'cue' placed around 'l' is 'CLUE'.
'such as' is the link.
(Other definitions for clue that I've seen before include "For example, this" , "Aid to solving" , "Evidence that helps to solve a problem" , "Hint, tip" , "Helpful hint" .)