Tune I like, yodelled at first in casual manner (6)
I believe the answer is:
airily
'in casual manner' is the definition.
(I know this)
'tune i like yodelled at first' is the wordplay.
'tune' becomes 'air' (air can mean a piece of music).
'at first' says to take the initial letters.
The first letters of 'like yodelled' is 'ly'.
'air'+'i'+'ly'='AIRILY'
(Other definitions for airily that I've seen before include "In a delicate manner" , "Flippantly, casually" , "In a light manner" , "Breezily" , "Cheerfully" .)