Serious; burdensome (5)
I believe the answer is:
heavy
I believe this is a double definition.
'serious' is the first definition.
(I've seen this in another clue)
'burdensome' is the second definition.
(similar in meaning)
(Other definitions for heavy that I've seen before include "Oppressive" , "Not light" , "Difficult to lift" , "Mosque prayer leader" , "Weighty - ponderous" .)