Horse heading for paddock that’s unlocked? (7)
I believe the answer is:
piebald
'horse' is the definition.
'heading for paddock that's unlocked?' is the wordplay.
'heading for' says to take the initial letters.
'that's' becomes 'ie'.
'unlocked?' becomes 'bald' (without locks of hair).
The initial letter of 'paddock' is 'p'.
'p'+'ie'+'bald'='PIEBALD'
(Other definitions for piebald that I've seen before include "Motley" , "(Of an animal) marked in two different colours" , "Marked in two different colours, especially black and white" , "(Horse) mottled with two colours" , "Bicoloured" .)