Praise when got at nuclear explosion (12)
I believe the answer is:
congratulate
'praise' is the definition.
(I know that congratulate is a type of praise)
'got at nuclear explosion' is the wordplay.
'explosion' indicates an anagram.
'got'+'at'+'nuclear'='gotatnuclear'
'gotatnuclear' anagrammed gives 'CONGRATULATE'.
'when' is the link.
(Other definitions for congratulate that I've seen before include "Wish joy to" , "Praise (someone) for an achievement" , "Compliment" .)