Tummy trouble in dreadful days without drink (9)
I believe the answer is:
dyspepsia
'tummy trouble' is the definition.
'dreadful days without drink' is the wordplay.
'dreadful' indicates anagramming the letters.
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'drink' becomes 'pepsi' (Pepsi is an example**).
'days' is an anagram of 'dysa'.
'dysa' enclosing 'pepsi' is 'DYSPEPSIA'.
'in' acts as a link.
(Other definitions for dyspepsia that I've seen before include "Stomach problem" , "stomach complaint" , "it could result from rushing courses" , "Indigestion" , "Stomach upset - spied a spy (anag)" .)