Sheepdog reportedly moves unsteadily, having upset stomach (12)
I believe the answer is:
collywobbles
'upset stomach' is the definition.
(I've seen this before)
'sheepdog reportedly moves unsteadily' is the wordplay.
'sheepdog' becomes 'collie' (collie is a kind of sheepdog).
'reportedly' shows a homophone (sound like).
'moves unsteadily' becomes 'wobbles' ('wobble' can be a synonym of 'move unsteadily').
'collie' sounds like 'colly'.
'colly'+'wobbles'='COLLYWOBBLES'
'having' is the link.
(Other definitions for collywobbles that I've seen before include "Owls by Belloc (anag) -- severe gastroenteritis" , "Feeling of apprehension" , "Abdominal pain (causing feelings of apprehension?)" , "Upset stomach" , "Intense anxiety" .)