Special key needed for compound (4)
I believe the answer is:
salt
'compound' is the definition.
(salt is a kind of compound)
'special key needed' is the wordplay.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'key' becomes 'alt' (the Alt keyboard button).
'needed' means one lot of letters go next to another.
's'+'alt'='SALT'
'for' acts as a link.
(Other definitions for salt that I've seen before include "Preserve" , "Crystalline substance used to season food" , "Mariner; sodium chloride" , "Sea dog" , "Crystalline substance for the table" .)