Small bird has to dash (5)
I believe the answer is:
scoot
'to dash' is the definition.
('scoot' can be a synonym of 'dash')
'small bird' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'bird' becomes 'coot' (coot is a kind of bird).
's'+'coot'='SCOOT'
'has' is the link.
(Other definitions for scoot that I've seen before include "Skedaddle" , "hurry" , "hightail it" , "Leave quickly" , "Dart off" .)