Northern trade union mired in pretty abusive comments (9)
I believe the answer is:
contumely
'abusive comments' is the definition.
(I've seen this before)
'northern trade union mired in pretty' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'trade union' becomes 'TU' (abbreviation for trade union).
'mired in' is an insertion indicator (some letters get trapped or mired in others).
'pretty' becomes 'comely' (comely can mean pretty or attractive).
'n'+'tu'='ntu'
'ntu' put within 'comely' is 'CONTUMELY'.
(Another definition for contumely that I've seen is " abusive comments".)