Disastrously obese friend (7)
I believe the answer is:
fatally
'disastrously' is the definition.
Both the definition and answer are adverbs. Maybe there's an association between them I don't understand?
'obese friend' is the wordplay.
'obese' becomes 'fat' (similar in meaning).
'friend' becomes 'ally' (an ally is a friend or supporter).
'fat'+'ally'='FATALLY'
Can you help me to learn more?
(Other definitions for fatally that I've seen before include "with no hope of revival" , "in decisive manner" , "So as to cause death" , "Might one be wounded thus" , "producing disastrous result" .)