Agreed it’s fast in grass (8)
I believe the answer is:
relented
'agreed' is the definition.
The definition and answer can be both to do with fundamental existence as well as being past participle verbs.
Maybe there's an association between them I don't understand?
'fast in grass' is the wordplay.
'fast' becomes 'lent' (Lent is a period of fasting in Christianity).
'in' indicates putting letters inside.
'grass' becomes 'reed' (I've seen this before).
'lent' placed into 'reed' is 'RELENTED'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for relented that I've seen before include "Became less severe, let up" , "Gave in and became less severe" , "Became less intense" , "Gave in (to pleas?)" , "Gave in, softened" .)