Upset stomach, with bad days drinking cola (9)
I believe the answer is:
dyspepsia
'upset stomach' is the definition.
(synonyms)
'bad days drinking cola' is the wordplay.
'bad' 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'.
'with' acts as a link.
(Other definitions for dyspepsia that I've seen before include "Indigestion" , "Heartburn" , "internal upset" , "Stomach upset - spied a spy (anag)" , "stomach complaint" .)