Hires marine map-makers? (8)

I believe the answer is:

charters

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'hires' is the definition.
(I know this)

'marine map-makers?' is the wordplay.
'marine map' becomes 'chart' (I've seen this in another clue).
'makers?' becomes 'ers' (I can't justify this - if you can you should give a lot more credence to this answer).
'chart'+'ers'='CHARTERS'

Can you help me to learn more?

(Other definitions for charters that I've seen before include "Hires, a boat say" , "Hires a boat or plane" .)

I've seen this clue in the Evening Standard.

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