Stray into hill in panic (6)
I believe the answer is:
terror
'panic' is the definition.
(I know that panic can be written as terror)
'stray into hill' is the wordplay.
'stray' becomes 'err' (to err is to stray or go wrong).
'into' indicates putting letters inside.
'hill' becomes 'tor' (tor is a kind of hill).
'err' put inside 'tor' is 'TERROR'.
'in' is the link.
(Other definitions for terror that I've seen before include "State of extreme fright" , "Paralysing fear" , "Overwhelming feeling of fear" , "Abject fear" , "Fear - now being warred on" .)