Armstrong has stomach upset (7)
I believe the answer is:
satchmo
'armstrong' is the definition.
(I know that armstrong can be written as Satchmo)
'stomach upset' is the wordplay.
'upset' is an anagram indicator.
'stomach' anagrammed gives 'SATCHMO'.
'has' is the link.
(Other definitions for satchmo that I've seen before include "trumpeter" , "Jazz musician" , "Louis Armstrong's nickname" , "Name used by jazz player Louis Armstrong" , "Louis Armstrong's byname" .)