Light cereal grass circled by insect (6)

'light' is the definition.
('floaty' can be similar in meaning to 'light')

'cereal grass circled by insect' is the wordplay.
'cereal grass' becomes 'oat' (oat is a kind of cereal grass).
'circled by' means one lot of letters goes inside another (I've seen this in other clues).
'insect' becomes 'fly' (fly is a kind of insect).
'oat' put inside 'fly' is 'FLOATY'.