I nearly forgot about one very dodgy dealer (4)
I believe the answer is:
spiv
'dodgy dealer' is the definition.
(I've seen this before)
'i nearly forgot about one very' is the wordplay.
'i nearly forgot' becomes 'PS' (as in a PS in a letter).
'about' says the letters should be written in reverse.
'one' becomes 'i' (Roman numeral).
'very' becomes 'v' (common abbreviation).
'ps' backwards is 'sp'.
'sp'+'i'+'v'='SPIV'
(Other definitions for spiv that I've seen before include "Shady salesman" , "Flashily dressed dodgy dealer" , "Swindler" , "Flashy black-marketeer" , "WW two black-marketeer" .)