Discernment shows gift horse to be phoney (9)
I believe the answer is:
foresight
'discernment' is the definition.
(foresight is a kind of discernment)
'gift horse to be phoney' is the wordplay.
'to be phoney' indicates anagramming the letters.
'gift'+'horse'='gifthorse'
'gifthorse' with letters rearranged gives 'FORESIGHT'.
'shows' acts as a link.
(Other definitions for foresight that I've seen before include "Caution" , "Vision" , "Power of prediction" , "Anticipation" , "Planning" .)