Slanderous attacks must keep them oddly subdued (5)
I believe the answer is:
muted
'subdued' is the definition.
(both can mean restrained or subtle)
'slanderous attacks must keep them oddly' is the wordplay.
'slanderous attacks' becomes 'mud' (as in dragging someone's name through the mud).
'must keep' means one lot of letters goes inside another.
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
The alternate letters of 'them' are 'te'.
'mud' placed around 'te' is 'MUTED'.
(Other definitions for muted that I've seen before include "In music, with sound deadened or muffled" , "Of great weight" , "with reduced volume" , "Faint" , "Quiet or muffled" .)