Left impressed by manners flipping infrequently (6)
I believe the answer is:
seldom
'infrequently' is the definition.
(I've seen this in another clue)
'left impressed by manners flipping' is the wordplay.
'left' becomes 'l' (common abbreviation).
'impressed by' means one lot of letters goes inside another.
'manners' becomes 'modes' (manners or modes of doing something).
'flipping' says the letters should be written backwards.
'l' placed into 'modes' is 'modles'.
'modles' backwards is 'SELDOM'.
(Other definitions for seldom that I've seen before include "Once in a blue moon" , "Not 21" , "Alluvium" , "Not often, infrequently" , "Only occasionally" .)