Calling upon one in employment (10)

I believe the answer is:

invocation

'calling' is the definition.
(thesaurus)

'one in employment' is the wordplay.
'employment' becomes 'vocation'.
'in'+'vocation'='INVOCATION'

'upon' acts as a link.

(Other definitions for invocation that I've seen before include "appeal for inspiration" , "Incantation" .)

I've seen this clue in The Globe and Mail.

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