Bulky; onerous (7)
I believe the answer is:
weighty
This clue is a double definition.
'bulky' is the first definition.
(thesaurus)
'onerous' is the second definition.
(thesaurus)
(Other definitions for weighty that I've seen before include "Heavy - very serious" , "Cumbersome" , "substantial" , "having great influence" , "significant" .)