Wild horse regretted biting (6)
I believe the answer is:
rugged
'wild' is the definition.
(rugged can describe wild terrain)
'horse regretted biting' is the wordplay.
'horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'regretted' becomes 'rued' ('rue' can be a synonym of 'regret').
'biting' is an insertion indicator (some letters bite or swallow others).
'gg' placed within 'rued' is 'RUGGED'.
(Other definitions for rugged that I've seen before include "Tough, craggy, sturdy and strong" , "wearing a wig?" , "Having toughness and determination" , "(Of a man) having attractively strong and rough features" , "Rocky and steep" .)