Tattered duster has unionist in stitches (7)
I believe the answer is:
sutured
'in stitches' is the definition.
(I've seen this in another clue)
'tattered duster has unionist' is the wordplay.
'tattered' is an anagram indicator (a tattered or broken form of the letters).
'has' means one lot of letters goes inside another.
'unionist' becomes 'u' (abbreviation as in Democratic Unionist Party).
'duster' anagrammed gives 'sutred'.
'sutred' placed around 'u' is 'SUTURED'.
(Other definitions for sutured that I've seen before include "Rust due {anag.)" , "stitched up" .)