Piles of grass chimneys (9)
I believe the answer is:
haystacks
'piles' is the definition.
(haystack is a kind of pile)
'grass chimneys' is the wordplay.
'grass' becomes 'hay' (I have seen 'Mown grass ' mean 'hay' so perhaps 'grass' could also mean 'hay').
'chimneys' becomes 'stacks' (stack is a kind of chimney).
'hay'+'stacks'='HAYSTACKS'
'of' acts as a link.
(Other definitions for haystacks that I've seen before include "It's hard to find needles in these" , "Giant wrestler" , "country piles" .)