Racket’s special type of shaft (4)
I believe the answer is:
scam
'racket's' is the definition.
(racket is a type of scam or scheme)
'special type of shaft' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'type of shaft' becomes 'cam' (I've seen this in another clue).
's'+'cam'='SCAM'
(Other definitions for scam that I've seen before include "Fraudulent action" , "Racket" , "A modern kind of swindle" , "Fraudulent business scheme" , "Con trick" .)