Unknown, some screwball (4)
I believe the answer is:
zany
'screwball' is the definition.
(I've seen this before)
'unknown some' is the wordplay.
'unknown' becomes 'z' (algebra).
'some' becomes 'any' ('any' can be similar in meaning to 'some').
'z'+'any'='ZANY'
(Other definitions for zany that I've seen before include "Amusingly idiosyncratic" , "Bizarrely comic" , "Amusingly odd" , "Comically idiotic" , "Absurd" .)