Troops set back a party sign blocking way (4,2)

I believe the answer is:

road up

'sign blocking way' is the definition.
(road sign indicating a road has been taken up)

'troops set back a party' is the wordplay.
'troops' becomes 'or' (military abbreviation for other ranks).
'set back' says the letters should be written backwards.
'party' becomes 'dup' (Democratic Unionist Party).
'or' reversed gives 'ro'.
'ro'+'a'+'dup'='ROAD UP'

I've seen this clue in The Guardian.

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