Officers not the first choice to convert Poles (10)

I believe the answer is:

subalterns

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

'not the first choice to convert poles' is the wordplay.
'not the first choice' becomes 'sub' (abbreviation for substitute e.g. a reserve player).
'to convert' becomes 'alter' (I've seen this before).
'poles' becomes 'ns' (North and South poles).
'sub'+'alter'+'ns'='SUBALTERNS'

I've seen this clue in The Guardian.

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