Coarse head of corn starts to turn healthily yellow (6)
I believe the answer is:
earthy
'coarse' is the definition.
('earthy' can be a synonym of 'coarse')
'head of corn starts to turn healthily yellow' is the wordplay.
'head of corn' becomes 'ear' (I've seen this before).
'starts to' indicates taking the first letters.
The initial letters of 'turn healthily yellow' is 'thy'.
'ear'+'thy'='EARTHY'
(Other definitions for earthy that I've seen before include "Blue" , "Dirty" , "Natural" , "Coarse or gross" , "Gross, like the soil" .)