Reasonable start for folk song (4)
I believe the answer is:
'reasonable' is the definition.
(I know that reasonable can be written as fair)
'start for folk song' is the wordplay.
'start for' says to take the initial letters.
'song' becomes 'air' (air can mean a piece of music).
The first letter of 'folk' is 'f'.
(Other definitions for fair that I've seen before include "gorgeous" , "Just - blonde - outdoor entertainment" , "[OPPOSITE OF] Dark" , "(Of skin) pale" , "Just; beautiful; blond" .)