Replaced scythe, originally ok, cuts some grass (7)
I believe the answer is:
tussock
'some grass' is the definition.
(I've seen this before)
'replaced scythe originally ok cuts' is the wordplay.
'replaced' is an anagram indicator.
'originally' suggests taking the first letters.
The initial letter of 'scythe' is 's'.
's'+'ok'+'cuts'='sokcuts'
'sokcuts' anagrammed gives 'TUSSOCK'.
(Other definitions for tussock that I've seen before include "Clump of hair" , "Bushy clump of grass" , "Dense clump of grass - stuck so (anag)" , "Grassy clump" , "(Of grass) cut" .)