Sea to surround large computer (9)

I believe the answer is:

mainframe

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'large computer' is the definition.
(I've seen this before)

'sea to surround' is the wordplay.
'sea' becomes 'main' (synonyms).
'to surround' becomes 'frame' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'main'+'frame'='MAINFRAME'

Can you help me to learn more?

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

I've seen this clue in The Daily Express.

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