Gay (6)
'gay' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for blithe that I've seen before include "Innocently happy" , "'Hail to thee, ,,,,,, spirit'- Shelley (6)" , "Shelley: 'Hail to thee . . . . . . spirit! Bird thou never wert'" , "With neither worries nor cares" , "Happy, without worries or cares" .)