Stomach upset - unpleasant ailment lovers originally shared (6)
I believe the answer is:
mutual
'shared' is the definition.
('mutual' can be similar in meaning to 'shared')
'stomach upset unpleasant ailment lovers originally' is the wordplay.
'stomach' becomes 'tum' ('tum' can be a synonym of 'stomach').
'upset' says the letters should be written backwards.
'originally' says to take the initial letters.
The initial letters of 'unpleasant ailment lovers' is 'ual'.
'tum' written backwards gives 'mut'.
'mut'+'ual'='MUTUAL'
(Other definitions for mutual that I've seen before include "Held in common" , "Reciprocal or shared, like friendship" , "Dickens novel 'Our . . . . . . Friend'" , "building society, say" , "Common to two or more" .)