David and Jack perhaps who took out bigger rivals? (5-7)

I believe the answer is:

giant-killers

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'who took out bigger rivals?' is the definition.
I don't know anything about this answer so I can't tell whether it can be defined by this definition.

'david and jack' is the wordplay.
'david' becomes 'giant killer' (I've seen this before).
'and' means one lot of letters go next to another.
'jack' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'giantkiller'+'s'='GIANT-KILLERS'

'perhaps' acts as a link.

Can you help me to learn more?

I've seen this clue in The Irish Times.

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