Workers organised party on river for Mary? (5)
I believe the answer is:
tudor
'for mary?' is the definition.
(I've seen this before)
'workers organised party on river' is the wordplay.
'workers organised' becomes 'TU' (abbreviation for trade union).
'party' becomes 'do' (do is a kind of party).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'river' becomes 'r' (abbreviation for river).
'tu'+'do'+'r'='TUDOR'
(Other definitions for tudor that I've seen before include "Name of English royal family during 16th century" , "A royal house of England" , "Royal house of Henry VIII" , "Name of a royal house" , "Half-timbered-house style" .)