Urn thrown inside that is to harden (5)
I believe the answer is:
inure
'harden' is the definition.
('inure' can be a synonym of 'harden')
'urn thrown inside that is' is the wordplay.
'thrown' indicates an anagram.
'inside' indicates putting letters inside.
'that is' becomes 'ie' ('i.e.' can be a synonym of 'that is').
'urn' with letters rearranged gives 'nur'.
'nur' placed inside 'ie' is 'INURE'.
'to' acts as a link.
(Other definitions for inure that I've seen before include "Come to accept" , "Harden against, make tough" , "Harden, habituate" , "harden attitude" , "Accustom (someone) to something unpleasant" .)