Legs covered in grass got worse (8)
I believe the answer is:
relapsed
'got worse' is the definition.
('relapse' can be a synonym of 'get worse')
'legs covered in grass' is the wordplay.
'legs' becomes 'laps' (I am not sure about this - if you are sure you should believe this answer much more).
'covered in' means one lot of letters goes inside another.
'grass' becomes 'reed' (I've seen this before).
'laps' put within 'reed' is 'RELAPSED'.
Can you help me to learn more?
(Other definitions for relapsed that I've seen before include "Regressed, got worse" , "Deteriorated in health again" , "Returned to former state, of illness or wicked ways" , "Fell back again or backslid" , "Returned to a poorer state of health" .)