Days sadly drinking cola causing indigestion (9)
I believe the answer is:
dyspepsia
'indigestion' is the definition.
(synonyms)
'days sadly drinking cola' is the wordplay.
'sadly' indicates an anagram.
'drinking' indicates putting letters inside (inserted letters are consumed or drunk).
'cola' becomes 'Pepsi' (cola brand).
'days' with letters rearranged gives 'dysa'.
'dysa' going around 'pepsi' is 'DYSPEPSIA'.
'causing' is the link.
(Other definitions for dyspepsia that I've seen before include "stomach complaint" , "it could result from rushing courses" , "Stomach problem" , "Indigestion" , "Stomach upset - spied a spy (anag)" .)