Affected little devil wasn't sincere (8)
I believe the answer is:
impacted
'affected' is the definition.
('impact' can be a synonym of 'affect')
'little devil wasn't sincere' is the wordplay.
'little devil' becomes 'imp' (I've seen this before).
'wasn't sincere' becomes 'acted' (acting rather than being sincere).
'imp'+'acted'='IMPACTED'
(Other definitions for impacted that I've seen before include "Wedged firmly" , "had influence" , "Crushed" , "Had an effect upon" , "Wisdom teeth may be so stuck" .)