Computer firm with a name that is unusual (9)
I believe the answer is:
mainframe
'computer' is the definition.
(I know that mainframe is a type of digital computer)
'firm with a name that is unusual' is the wordplay.
'with' means one lot of letters go next to another.
'that is unusual' indicates an anagram (I've seen 'that's unusual' mean this).
'firm'+'a'+'name'='firmaname'
'firmaname' anagrammed gives 'MAINFRAME'.
(Other definitions for mainframe that I've seen before include "Mar famine (anag.)" , "Large central computer" , "Computer's central processing unit" , "Principal computer" , "powerful computer" .)