This learner's stopping signal (4)
I believe the answer is:
clue
'this' is the definition.
(the clue referring to itself!)
'learner's stopping signal' is the wordplay.
'learner' becomes 'l' (as in a learner driver with L-plates on their car).
'stopping' is an insertion indicator (inserted letters stop or plug a gap).
'signal' becomes 'cue' (as in an actor's cue).
'l' put within 'cue' is 'CLUE'.
(Other definitions for clue that I've seen before include "One indicates" , "Helpful hint" , "Luce (anag.)" , "Crossword item" , "contribution to crossword" .)