Shell for mad person? (7)
I believe the answer is:
nutcase
This clue is a double definition.
'shell' is the first definition.
(I've seen this before)
'mad person?' is the second definition.
(I've seen this before)
'for' is the link.
(Other definitions for nutcase that I've seen before include "Psycho" , "Slangily, a crazy person" , "Crackpot" , "bananas" , "He's mad" .)