Double over after endlessly happy praise (7)
I believe the answer is:
applaud
'praise' is the definition.
(I know that applaud is a more specific form of the action praise)
'double over after endlessly happy' is the wordplay.
'double' becomes 'dual' (synonyms).
'over' says the letters should be written backwards.
'after' means one lot of letters go next to another.
'endlessly' means to look at the middle letters (the ends of the words are removed).
The central letters of 'happy' are 'app'.
'dual' written backwards gives 'laud'.
'laud' put after 'app' is 'APPLAUD'.
(Other definitions for applaud that I've seen before include "Approve with clapping" , "Express approval of a performance" , "Reward for performance" , "Clap (a performance)" , "Cheer" .)