Can't use terribly eccentric person (7)
I believe the answer is:
nutcase
'eccentric person' is the definition.
(nutcase is a kind of eccentric person)
'can't use terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'cant'+'use'='cantuse'
'cantuse' anagrammed gives 'NUTCASE'.
(Other definitions for nutcase that I've seen before include "Maniac - shell" , "bird may need to crack this" , "Psycho" , "A crazy person like fruit carapace" , "He's mad" .)