Partly avoid a horse in the Gem State (5)
I believe the answer is:
idaho
'the gem state' is the definition.
(Idaho is nicknamed the Gem State)
'partly avoid a horse' is the wordplay.
'partly' indicates a hidden word (part of the clue is used).
'IDAHO' is hidden in the letters of 'avoid a horse'.
'in' is the link.
(Other definitions for idaho that I've seen before include "My Own Private -- (1991 film)" , "Union member?" , "One of 22 Across, capital Boise" , "American state, capital Boise (5)" , "State of north-west USA" .)