A small bird is livid (5)
I believe the answer is:
ashen
'livid' is the definition.
('ashen' can be a synonym of 'livid')
'a small bird' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'bird' becomes 'hen' (hen is a kind of bird**).
'a'+'s'+'hen'='ASHEN'
'is' is the link.
(Other definitions for ashen that I've seen before include "Anaemic looking" , "Pale (with shock/illness)" , "Anaemic-looking" , "Grey in colour" , "Unhealthily pale" .)