Went down in action involving special constable before finale (9)
I believe the answer is:
descended
'went down' is the definition.
('descend' can be a synonym of 'go down')
'action involving special constable before finale' is the wordplay.
'action' becomes 'deed' (deed is a kind of action).
'involving' means one lot of letters goes inside another.
'special constable' becomes 'SC' (abbreviation for special constable).
'before' means one lot of letters go next to another.
'finale' becomes 'end' (I've seen this before).
'sc'+'end'='scend'
'deed' going around 'scend' is 'DESCENDED'.
'in' is the link.
(Other definitions for descended that I've seen before include "went downhill?" , "Came down" .)