Days of old power studied for big corporation (6-3,6)
I believe the answer is:
middle-age spread
'corporation' is the definition.
(both are informal terms for body fat)
'days of old power studied for big' is the wordplay.
I cannot quite see how this works, but
'days' could be 'd' and 'd' is located in the answer.
'power' could be 'id' (id is a kind of power) and 'id' is found within the answer.
'studied' could be 'read' (I've seen this before) and 'read' is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for middle-age spread that I've seen before include "Weight gain in later life" , "Pot" , "Weight problem for those growing older" .)