Appear expert on white horses? (7)
I believe the answer is:
surface
'appear' is the definition.
(I know that surface is a type of appear)
'expert on white horses?' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'on' means one lot of letters go next to another.
'white horses?' becomes 'surf' (I can't justify this - if you can you should give a lot more credence to this answer).
'ace' put after 'surf' is 'SURFACE'.
(Other definitions for surface that I've seen before include "emerge?" , "Outside; rise to the top" , "Come up for air" , "Superficial appearance" , "skin" .)