Sign of a river that is small (5)
I believe the answer is:
aries
'sign' is the definition.
(star sign)
'a river that is small' is the wordplay.
'river' becomes 'r' (abbreviation).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'small' becomes 's' (abbreviation - e.g. clothes size).
'a'+'r'+'ie'+'s'='ARIES'
'of' is the link.
(Other definitions for aries that I've seen before include "House" , "Zodiacal sign also known as the Ram" , "Constellation with sign of the ram" , "Stars" , "star group" .)