Ponderous; oppressive (5)
I believe the answer is:
heavy
This clue is a double definition.
'ponderous' is the first definition.
(I know that ponderous can be written as heavy)
'oppressive' is the second definition.
(similar in meaning)
(Other definitions for heavy that I've seen before include "Oppressive" , "Hard to lift" , "Of great weight" , "Villain" , "Stodgy; difficult" .)