Shell a silly fellow (7)
I believe the answer is:
nutcase
'shell' is the definition.
(I've seen this before)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for nutcase that I've seen before include "He's mad" , "one's cracked shell?" , "Slang for a crazy person" , "Maybe almond covering" , "One may finish in the madhouse" .)