Knut will soften spleen (3,6)
I believe the answer is:
ill temper
'spleen' is the definition.
(thesaurus)
'knut will soften' is the wordplay.
'knut will' becomes 'I'll' (where the crossword setter is 'Knut').
'soften' becomes 'temper' (I've seen this in another clue).
'ill'+'temper'='ILL TEMPER'
(Other definitions for ill temper that I've seen before include "Surliness" , "Irritability" , "black humour" .)