Water and grass in London park (7)
I believe the answer is:
hydrate
'water' is the definition.
(I've seen this before)
'grass in london park' is the wordplay.
'grass' becomes 'rat' (both can mean a snitch or informer).
'in' is an insertion indicator.
'london park' becomes 'hyde' (I've seen this before).
'rat' put inside 'hyde' is 'HYDRATE'.
'and' is the link.
(Other definitions for hydrate that I've seen before include "Give water" , "Apply water" , "Thready (anag.)" , "Cause (something) to absorb water" , "Moisturise" .)