It's dull outside, terribly (7)
I believe the answer is:
tedious
'it's dull' is the definition.
(similar in meaning)
'outside terribly' is the wordplay.
'terribly' indicates an anagram.
'outside' is an anagram of 'TEDIOUS'.
(Other definitions for tedious that I've seen before include "Banal" , "Long, slow, dull" , "drab" , "Outside (anag.)" , "Dull as ditchwater" .)