Ill-mannered unionist among group is close to Taoiseach (7)
I believe the answer is:
loutish
'ill-mannered' is the definition.
(I've seen this before)
'unionist among group is close to taoiseach' is the wordplay.
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'among' means one lot of letters goes inside another.
'group' becomes 'lot' (lot is a kind of group**).
'close to' indicates one should take the final letters (the close/ending of).
The last letter of 'taoiseach' is 'h'.
'u' put into 'lot' is 'lout'.
'lout'+'is'+'h'='LOUTISH'
(Other definitions for loutish that I've seen before include "like a hooligan" , "Ill-mannered and coarse, behaviour say" , "Boorish" , "uncouth and aggressive" .)