Dope beginning to disturb a stomach (5)
I believe the answer is:
datum
'dope' is the definition.
(thesaurus)
'beginning to disturb a stomach' is the wordplay.
'beginning to' indicates taking the first letters.
'stomach' becomes 'tum' ('tum' can be a synonym of 'stomach').
The first letter of 'disturb' is 'd'.
'd'+'a'+'tum'='DATUM'
(Other definitions for datum that I've seen before include "Given item" , "Given piece of information" , "bit of info" , "Bit of dope" , "Little bit of information" .)