Furtive sort of hat style (8)
I believe the answer is:
stealthy
'furtive' is the definition.
('stealthy' can be a synonym of 'furtive')
'sort of hat style' is the wordplay.
'sort of' is an anagram indicator.
'hat'+'style'='hatstyle'
'hatstyle' with letters rearranged gives 'STEALTHY'.
(Other definitions for stealthy that I've seen before include "'Sneaky, clandestine (8)'" , "Covert - clandestine" , "Secretive, silent" , "Underhand" , "Acting in quiet or secret way" .)