This is one prompt to pen line (4)
I believe the answer is:
clue
'this' is the definition.
(the clue referring to itself!)
'one prompt to pen line' is the wordplay.
'one prompt' becomes 'cue' (cue can mean a prompt or signal).
'to pen' is an insertion indicator.
'line' becomes 'l' (used when specifying particular lines from a poem).
'cue' enclosing 'l' is 'CLUE'.
'is' acts as a link.
(Other definitions for clue that I've seen before include "Hint, tip" , "contribution to crossword" , "Aid to solving" , "Hint; piece of evidence" , "crossword guide" .)