See uncle about a dirty trick! (4)
I believe the answer is:
scam
'a dirty trick' is the definition.
'scam' can be an answer for 'trick' (I've seen this in other clues). I am unsure of the 'a dirty' bit.
'see uncle about' is the wordplay.
'see' becomes 'C' (the word for the letter, according to Chambers).
'uncle' becomes 'sam' (Uncle Sam is an example).
'about' is an insertion indicator.
'c' placed into 'sam' is 'SCAM'.
Can you help me to learn more?
(Other definitions for scam that I've seen before include "Racket" , "Screw" , "Swindle (Slang)" , "Fraudulent scheme, swindle" , "An informal swindle" .)