Terribly full in a way, perhaps (7)
I believe the answer is:
awfully
'terribly' is the definition.
(synonyms)
'full in a way perhaps' is the wordplay.
'in' means one lot of letters goes inside another.
'perhaps' indicates anagramming the letters.
'way' with letters rearranged gives 'awy'.
'full' placed inside 'awy' is 'AWFULLY'.
(Other definitions for awfully that I've seen before include "Frightfully" , "A lot" , "Jolly" , "Terribly" , "Very much" .)