Why I get terribly heavy (7)
I believe the answer is:
weighty
'heavy' is the definition.
(synonyms)
'why i get terribly' is the wordplay.
'terribly' is an anagram indicator.
'why'+'i'+'get'='whyiget'
'whyiget' is an anagram of 'WEIGHTY'.
(Other definitions for weighty that I've seen before include "Cumbersome" , "having great influence" , "significant" , "Important" , "Heavy - very serious" .)