Horse has groom removing marks (3)
I believe the answer is:
cob
'horse' is the definition.
(type of horse)
'groom removing marks' is the wordplay.
'groom' becomes 'comb' (combing is a way of grooming oneself).
'removing' is a deletion indicator.
'marks' becomes 'M' (former German currency).
'comb' with 'm' taken away is 'COB'.
'has' acts as a link.
(Other definitions for cob that I've seen before include "A loaf; a nut; a horse" , "Hazelnut" , "Horse, male swan and loaf of bread" , "mount" , "mixture of clay and straw" .)