Horse beginning to panic -- that is plain (7)
I believe the answer is:
piebald
'horse' is the definition.
'beginning to panic that is plain' is the wordplay.
'beginning to' suggests taking the first letters.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'plain' becomes 'bald' (both can mean unadorned).
The initial letter of 'panic' is 'p'.
'p'+'ie'+'bald'='PIEBALD'
(Other definitions for piebald that I've seen before include "(Of horse) patched with two colours" , "(Of an animal) marked in two different colours" , "Motley" , "(Of horse) black and white in patches" , "Of horse, with patches of white and dark colour" .)