A head turning to a starchy tuber (6)
I believe the answer is:
potato
'a starchy tuber' is the definition.
(I've seen this in another clue)
'a head turning to' is the wordplay.
'head' becomes 'top' (synonyms).
'turning' is a reversal indicator.
'a'+'top'='atop'
'atop' back-to-front is 'pota'.
'pota'+'to'='POTATO'
(Other definitions for potato that I've seen before include "Tuber, popular in Ireland" , "Vegetable with eyes" , "The humble spud" , "Plant tuber eaten as a vegetable" , "The vegetable is just too pat" .)