Cheat who works with a pack (4,5)
I believe the answer is:
card sharp
'cheat' is the definition.
(I've seen this before)
'works with a pack' is the wordplay.
'works' becomes 'harp' (I can't justify this - if you can you should believe this answer much more).
'with' says to put letters next to each other.
'a pack' becomes 'cards' (pack of cards).
'harp' after 'cards' is 'CARD-SHARP'.
'who' is the link.
(Other definitions for card sharp that I've seen before include "one manipulating clubs?" , "Cheating gambler" , "You won't get a good deal out of one" , "manipulating the pack?" , "Trickster" .)