One who’s cunning by filling seats from the back (8)
I believe the answer is:
slyboots
'one who's cunning' is the definition.
(name for a sly person)
'filling seats from the back' is the wordplay.
'filling' means one lot of letters goes inside another.
'seats' becomes 'stools' (stool is a kind of seat).
'from' becomes 'by' (by can mean from a particular person).
'the back' says the letters should be written in reverse.
'stools' going around 'by' is 'stoobyls'.
'stoobyls' backwards is 'SLYBOOTS'.
'by' is the link.
(Other definitions for slyboots that I've seen before include "cunning chap" , "One who's deep" , "wily type" .)