Smartens himself with an upward sneer quietly (6)
I believe the answer is:
preens
'smartens himself' is the definition.
I can't judge whether this defines the answer.
'an upward sneer quietly' is the wordplay.
'an upward' says the letters should be written backwards (in a down clue, letters go up).
'quietly' becomes 'p' (p for piano and quiet).
'sneer'+'p'='sneerp'
'sneerp' in reverse letter order is 'PREENS'.
'with' is the link.
Can you help me to learn more?
(Other definitions for preens that I've seen before include "Titivates" , "Grooms oneself in a vain manner" , "Grooms, as a bird would" , "Arranges as a bird does its feathers" , "Trims feathers with beak" .)