Crossword Genius

Instruct colleague in the vernacular (10)

Ross

I believe the answer is:

informally

'in the vernacular' is the definition.

'instruct colleague' is the wordplay.
'instruct' becomes 'inform'.
'colleague' becomes 'ally' (I've seen this before).
'inform'+'ally'='INFORMALLY'

(Other definitions for informally that I've seen before include "in relaxed fashion" , "in relaxed manner" , "off the record" .)

I've seen this clue in The Times.
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