The horse is good – not frisky (5)
I believe the answer is:
pinto
'the horse' is the definition.
(pinto is a kind of horse)
'good not frisky' is the wordplay.
'good' becomes 'pi' (I've seen this in other clues**).
'frisky' is an anagram indicator.
'not' with letters rearranged gives 'nto'.
'pi'+'nto'='PINTO'
'is' acts as a link.
(Other definitions for pinto that I've seen before include "Mount" , "Spotted or piebald horse" , "H [HORSE]" , "Piebald horse (US)" , "patchy horse" .)