Sell hard One Week (4)
I believe the answer is:
hawk
'sell' is the definition.
(hawking is a kind of selling)
'hard one week' is the wordplay.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'one' becomes 'a'.
'week' becomes 'wk' (common abbreviation).
'h'+'a'+'wk'='HAWK'
(Other definitions for hawk that I've seen before include "27 [SPIT]" , "work as door-to-door salesman" , "Aggressive person" , "someone for fighting" , "Flog" .)