Instruct colleague in the vernacular (10)
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" .)