A double round to celebrate (4)
I believe the answer is:
laud
'to celebrate' is the definition.
(I've seen this before)
'a double round' is the wordplay.
'a double' becomes 'dual' (I've seen this in another clue).
'round' says the letters should be written backwards.
'dual' back-to-front is 'LAUD'.
(Other definitions for laud that I've seen before include "'Praise, glorify (4)'" , "Praise highly" , "Praise, extol" , "Praiseworthy" , "Archbishop of Canterbury, executed 1645" .)