A trick with never-ending application (4)
I believe the answer is:
ruse
'a trick' is the definition.
(I've seen this before)
'with never-ending application' is the wordplay.
'with' says to put letters next to each other.
'ending' says to take the final letters.
'application' becomes 'use' (I've seen this in other clues).
The final letter of 'never' is 'r'.
'r'+'use' is 'RUSE'.
(Other definitions for ruse that I've seen before include "Sure to be a trick" , "Sure its a strange deception" , "Deceptive manoeuvre" , "Cunning plan" , "Subterfuge" .)