Little Ronald has a horse (4)
I believe the answer is:
roan
'horse' is the definition.
(roan is a kind of horse)
'little ronald has a' is the wordplay.
'little' means to remove the last letter (the word is not quite complete).
'ronald' becomes 'ron' (I've seen this in other clues).
'has' says to put letters next to each other.
'a' becomes 'an'.
'ron' with its last letter removed is 'ro'.
'ro'+'an'='ROAN'
(Other definitions for roan that I've seen before include "Horse with brownish coat speckled with white" , "Bay horse with white/grey hairs in its coat" , "nag" , "Type of 17's coat colouring" , "Two-colour horse" .)