Devious devil, one sort of agent briefly turning to wit (9)
I believe the answer is:
videlicet
'to wit' is the definition.
(both can mean 'namely' or 'that is to say')
'devious devil one sort of agent briefly turning' is the wordplay.
'devious' indicates anagramming the letters.
'one' becomes 'I' (Roman numeral).
'sort of agent briefly' becomes 'tec' (short for detective; 'briefly' asks for an abbreviation).
'turning' is a reversal indicator.
'tec' back-to-front is 'cet'.
'devil' anagrammed gives 'videl'.
'videl'+'i'+'cet'='VIDELICET'
(Other definitions for videlicet that I've seen before include "to wit" , "Viz. in full, namely" , "that is to say" , "in other words" .)