Fishy expression - up to no good (4)
I believe the answer is:
pout
'fishy expression' is the definition.
'pout' can be an answer for 'expression' (I have seen 'Sullen expression ' mean 'pout' so perhaps 'expression' could also mean 'pout'). I'm unsure of the 'fishy' bit.
'up to no good' is the wordplay.
'no good' indicates anagramming the letters (no good can mean faulty or defective).
'up'+'to'='upto'
'upto' anagrammed gives 'POUT'.
Can you help me to learn more?
(Other definitions for pout that I've seen before include "set of lips" , "Sullen expression" , "Petulant expression" , "Sulky (or sexy?) expression" , "Express sulks with lips" .)