Hard at it, you won't be missing anything (3)
I believe the answer is:
hit
'won't be missing anything' is the definition.
I can't tell whether this defines the answer.
'hard at it' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'at' is a charade indicator (letters next to each other).
'h'+'it'='HIT'
'you' acts as a link.
Can you help me to learn more?
(Other definitions for hit that I've seen before include "Blow; popular song" , "A theatrical success" , "Chart topper" , "skier, perhaps" , "Successful record/film" .)