Celebrate twofold backing (4)
I believe the answer is:
laud
'celebrate' is the definition.
(I've seen this in another clue)
'twofold backing' is the wordplay.
'twofold' becomes 'dual' (synonyms).
'backing' is a reversal indicator.
'dual' backwards is 'LAUD'.
(Other definitions for laud that I've seen before include "Praise highly" , "Praise - celebrate" , "Archbishop of Canterbury, executed 1645" , "Praise, magnify" , "'Praise, glorify (4)'" .)