Little son acting badly and making false pretences (8)

'making false pretences' is the definition.
(to sham is to pretend or fake)

'little son acting badly and' is the wordplay.
'little' means to remove the last letter (the word is not quite complete).
'son' becomes 's'.
'acting badly' becomes 'hamming' (to ham is to over-act).
'and' becomes 'n' (common abbreviation for 'and').
's'+'hamming'+'n'='shammingn'
'shammingn' with its last letter taken off is 'SHAMMING'.