Big surprise in the west for senior officers (6)

I believe the answer is:

majors

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'officers' is the definition.
(major is a kind of officer)

'big surprise in the west for senior' is the wordplay.
'big' becomes 'major' (I've seen this before).
'surprise in the west for senior' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'major'+'s'='MAJORS'

Can you help me to learn more?

(Another definition for majors that I've seen is " Army officers".)

I've seen this clue in The Irish Times.

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