Sort of motor not allowed by directors (8)
I believe the answer is:
outboard
'sort of motor' is the definition.
(I know that outboard is a type of internal combustion engine)
'not allowed by directors' is the wordplay.
'not allowed' becomes 'out' (I've seen this before).
'by' is a charade indicator (letters next to each other).
'directors' becomes 'board' (board of directors).
'out'+'board'='OUTBOARD'
(Other definitions for outboard that I've seen before include "Engine attached to boat" , "Small boat's motor" , "Type of boat engine" , "Sort of motor" , "Engine external to the boat" .)