Stomach calmer, perhaps one manages to regroup (8)
I believe the answer is:
magnesia
'stomach calmer perhaps' is the definition.
(magnesia is used in stomach medicines)
'one manages to regroup' is the wordplay.
'one' becomes 'i' (Roman numeral).
'to regroup' indicates anagramming the letters (I've seen this in other clues).
'i'+'manages'='imanages'
'imanages' is an anagram of 'MAGNESIA'.
(Other definitions for magnesia that I've seen before include "Asian gem (anag) - milk of ___" , "Medicinal powder" , "Carbonate used as an antacid" , "laxative" , "Is Ma easing off the white mineral?" .)