King brought inside to finish tantrum (5)
I believe the answer is:
strop
'tantrum' is the definition.
'king brought inside to finish' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'brought inside' means one lot of letters goes inside another.
'to finish' becomes 'stop' ('stop' can be a synonym of 'finish').
'r' inserted within 'stop' is 'STROP'.
(Other definitions for strop that I've seen before include "Piece of leather on which razor is sharpened" , "Sharpen an old blade" , "temper tantrum" , "Razor-sharpener" , "Outburst" .)