Dirt around middle of step becomes softened (5)
I believe the answer is:
muted
'softened' is the definition.
(muting is a kind of softening)
'dirt around middle of step' is the wordplay.
'dirt' becomes 'mud' (mud is a kind of dirt).
'around' means one lot of letters goes inside another.
'middle of' indicates the central letters.
The middle letters of 'step' are 'te'.
'mud' placed around 'te' is 'MUTED'.
'becomes' is the link.
(Other definitions for muted that I've seen before include "(Of sound/colour) softened" , "Muffled or silent" , "Faint" , "Quiet or muffled" , "Quietened" .)