Gesticulate in White Horse (4)
I believe the answer is:
wave
This clue is a double definition.
'gesticulate' is the first definition.
(waving is a kind of gesticulating)
'white horse' is the second definition.
(I've seen this before)
'in' acts as a link.
(Other definitions for wave that I've seen before include "Drive forward" , "Hand gesture -- hair feature" , "Shake a hand in the air" , "Roller, move to and fro" , "Billow" .)