Swan runs away initially, seeing snake (5)
I believe the answer is:
cobra
'snake' is the definition.
(I've seen this before)
'swan runs away initially' is the wordplay.
'swan' becomes 'cob' (cob is a kind of swan).
'initially' suggests taking the first letters.
The first letters of 'runs away' is 'ra'.
'cob'+'ra'='COBRA'
'seeing' is the link.
(Other definitions for cobra that I've seen before include "one leading charmed life?" , "dangerous striker" , "hamadryad" , "Deadly snake called King" , "Sinuous creature" .)