Days spent drinking cola and getting indigestion (9)
I believe the answer is:
dyspepsia
'indigestion' is the definition.
(I know that indigestion can be written as dyspepsia)
'days spent drinking cola' is the wordplay.
'spent' indicates anagramming the letters.
'drinking' indicates putting letters inside (inserted letters are consumed or drunk).
'cola' becomes 'Pepsi' (cola brand).
'days' is an anagram of 'dysa'.
'dysa' going around 'pepsi' is 'DYSPEPSIA'.
'and getting' is the link.
(Other definitions for dyspepsia that I've seen before include "Stomach problem" , "It was enough to make any man sick" , "Stomach upset - spied a spy (anag)" , ""It gives me wind problem"" , "digestive problem" .)