Principal set up computer (9)

I believe the answer is:

mainframe

'computer' is the definition.
(mainframe is a kind of computer)

'principal set up' is the wordplay.
'principal' becomes 'main' (both can mean major).
'set up' becomes 'frame' (synonyms).
'main'+'frame'='MAINFRAME'

(Other definitions for mainframe that I've seen before include "Mar famine (anag.)" , "powerful computer" , "Computer system" , "Principal computer" , "Large central computer" .)

I've seen this clue in The Guardian.

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