Impressive sign of temper (5)
I believe the answer is:
stamp
This clue could be a double definition.
'impressive sign' is the first definition.
'stamp' can be an answer for 'sign' (stamp is a kind of sign). I am unsure of the 'impressive' bit.
'temper' is the second definition.
(thesaurus)
'of' acts as a link.
Can you help me to learn more?
(Other definitions for stamp that I've seen before include "Bring foot down hard" , "Postage paid label" , "Hallmark" , "squash?" , "Evidence of payment of postage duty" .)