Small computer over a fast one (4)
I believe the answer is:
scam
'a fast one' is the definition.
(I've seen this in another clue)
'small computer over' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'computer' becomes 'Mac' (Apple Mac computer).
'over' says the letters should be written backwards.
'mac' reversed gives 'cam'.
's'+'cam'='SCAM'
(Other definitions for scam that I've seen before include "A slangy swindle or fraud" , "fiddle" , "Con trick" , "Swindle, fraud (colloq.)" , "Dodgy business" .)