First light sees one charmed by swan (4)
I believe the answer is:
leda
'one charmed by swan' is the definition.
(figure of Greek myth)
'first light sees' is the wordplay.
'first' indicates taking the first letters.
'sees' becomes 'eda' (I can't justify this - if you can you should believe this answer much more).
The initial letter of 'light' is 'l'.
'l'+'eda'='LEDA'
(Other definitions for leda that I've seen before include "She was seduced by Zeus disguised as a swan" , "Zeus turned into a swan to seduce her" , "- - and the Swan (mythology)" , "small satellite of Jupiter" , "Mother of Helen and Pollux" .)