Be dutiful about deliberate corpulence (7)
I believe the answer is:
obesity
'corpulence' is the definition.
(I know that corpulence can be written as obesity)
'be dutiful about deliberate' is the wordplay.
'be dutiful' becomes 'obey' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'deliberate' becomes 'sit' (I can't explain this - if you can you should give a lot more credence to this answer).
'obey' enclosing 'sit' is 'OBESITY'.
Can you help me to learn more?
(Other definitions for obesity that I've seen before include "State of unhealthy overweight" , "Unhealthy excess of fatness" , "Fatness to great excess" , "Abnormal fatness" , "health issue" .)