Terrible days without drink, and indigestion! (9)
I believe the answer is:
dyspepsia
'indigestion' is the definition.
(I know that indigestion can be written as dyspepsia)
'terrible days without drink' is the wordplay.
'terrible' is an anagram indicator.
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'drink' becomes 'pepsi' (Pepsi is an example).
'days' anagrammed gives 'dysa'.
'dysa' going around 'pepsi' is 'DYSPEPSIA'.
'and' is the link.
(Other definitions for dyspepsia that I've seen before include "stomach complaint" , "It was enough to make any man sick" , "digestive problem" , "digestive complaint" , ""It gives me wind problem"" .)