The Speaker’s going to moderate surliness (3,6)
I believe the answer is:
ill temper
'surliness' is the definition.
(I've seen this before)
'the speaker's going to moderate' is the wordplay.
'the speaker' becomes 'I' ('I' refers to the person saying it).
'going to' becomes 'll' (as in he'll, we'll etc).
'moderate' becomes 'temper' (synonyms).
'i'+'ll'+'temper'='ILL TEMPER'
(Other definitions for ill temper that I've seen before include "Irritability" , "Surliness" , "black humour" .)