Circuits covered with grass deteriorated (8)
I believe the answer is:
relapsed
'deteriorated' is the definition.
(I've seen this before)
'circuits covered with grass' is the wordplay.
'circuits' becomes 'laps' ('lap' can be a synonym of 'circuit').
'covered with' indicates putting letters inside.
'grass' becomes 'reed' (I've seen this before).
'laps' inserted inside 'reed' is 'RELAPSED'.
(Other definitions for relapsed that I've seen before include "Suffered setback" , "Returned to former state, of illness or wicked ways" , "Sank back (into illness)" , "Regressed, got worse" , "Dropped back" .)