Jar goes in a rack (7)
I believe the answer is:
agonise
'rack' is the definition.
(I've seen this before)
'jar goes in a' is the wordplay.
'jar' indicates an anagram (I've seen 'jars' mean this (the letters must be shocked or jarred)).
'goes'+'in'+'a'='goesina'
'goesina' with letters rearranged gives 'AGONISE'.
(Other definitions for agonise that I've seen before include "Suffer torment" , "rack" , "Be very worried" , "Undergo great anguish" , "Be in torments of indecision" .)