To dad, some yams are fruit! (6)
I believe the answer is:
papaya
'fruit' is the definition.
(papaya is a kind of fruit)
'to dad some yams are' is the wordplay.
'to dad' becomes 'papa' ('papa' can be a synonym of 'dad').
'some' says to take the initial letters.
The initial letters of 'yams are' is 'ya'.
'papa'+'ya'='PAPAYA'
(Other definitions for papaya that I've seen before include "Large, green West Indian fruit" , "Oval melon-like tropical fruit" , "Caribbean fruit" , "Tropical fruit with pink flesh and black seeds" , "Fruit with orange flesh" .)