The first timepiece has returned to guard against two-timing (5,4,4)

I believe the answer is:

watch ones back

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'guard against two-timing' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'the first timepiece has returned' is the wordplay.
'the first' says to put letters next to each other.
'timepiece' becomes 'watch' (watch is a kind of timepiece).
'has' becomes 'ones' (I can't explain this - if you can you should give a lot more credence to this answer).
'returned' becomes 'back' (I've seen this in other clues).
'watch' put next to 'ones' is 'watchones'.
'watchones'+'back'='WATCH ONES BACK'

'to' is the link.

Can you help me to learn more?

I've seen this clue in the Sydney Morning Herald.

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