Celebrate when double comes up (4)
I believe the answer is:
laud
'celebrate' is the definition.
(thesaurus)
'double comes up' is the wordplay.
'double' becomes 'dual' ('dual' can be a synonym of 'double').
'comes up' says the letters should be written backwards.
'dual' back-to-front is 'LAUD'.
'when' is the link.
(Other definitions for laud that I've seen before include "Archbishop of Canterbury, executed 1645" , "homage" , "Praise formally" , "Give praise to" , "Name of archbishop; praise" .)