Hard at it, you'll find success (3)
I believe the answer is:
hit
'success' is the definition.
(a hit is a commercial success)
'hard at it' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'at' says to put letters next to each other.
'h'+'it'='HIT'
'you'll find' is the link.
(Other definitions for hit that I've seen before include "A theatrical success" , "suit" , "Successful stroke, conspicuous success or connection to a website" , "not to miss!" , "Successful record" .)