Completely applaud how the optimist behaves (10)
I believe the answer is:
cheerfully
'the optimist behaves' is the definition.
I can't tell whether this definition defines the answer.
'completely applaud' is the wordplay.
I cannot really understand how this works, but
'completely' could be 'fully' (in full) and 'fully' is found within the answer.
'applaud' could be 'cheer' (cheering is a kind of applauding) and 'cheer' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
'how' is the link.
Can you help me to learn more?
(Other definitions for cheerfully that I've seen before include "In happy fashion" , "In a happy manner" .)