Garment being worn by companion in Italian river (6)
I believe the answer is:
poncho
'garment' is the definition.
(poncho is a kind of garment)
'worn by companion in italian river' is the wordplay.
'worn by' becomes 'on'.
'companion' becomes 'ch' (I've seen this in other clues).
'in' is an insertion indicator.
'italian river' becomes 'po'.
'on'+'ch'='onch'
'onch' put within 'po' is 'PONCHO'.
'being' acts as a link.
(Other definitions for poncho that I've seen before include "Blanket-style cloak" , "Blanket-like cloth with centre slit, worn as cloak" , "apparel" , "South American garment" , "South American cape" .)