A French scout mistakenly embracing unionist that's smarmy (8)
I believe the answer is:
unctuous
'smarmy' is the definition.
(I've seen this before)
'a french scout mistakenly embracing unionist' is the wordplay.
'a french' becomes 'un' ('a' in French).
'mistakenly' indicates an anagram.
'embracing' indicates putting letters inside.
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'scout' with letters rearranged gives 'ctuos'.
'ctuos' placed around 'u' is 'ctuous'.
'un'+'ctuous'='UNCTUOUS'
'that's' acts as a link.
(Other definitions for unctuous that I've seen before include "Having an oily manner, smugly virtuous" , "Excessively ingratiating" , "Slimily ingratiating" , "Sycophantic - oily" , "Excessively flattering" .)