Account for old grassland (7)
I believe the answer is:
explain
'account for' is the definition.
(I've seen this before)
'old grassland' is the wordplay.
'old' becomes 'ex'.
'grassland' becomes 'plain' (I have seen 'Open grassland' mean 'plain' so perhaps 'grassland' could also mean 'plain').
'ex'+'plain'='EXPLAIN'
(Other definitions for explain that I've seen before include "Resolve" , "Give reasons for" , "Show the meaning of, make clear" , "Elucidate, make clear" , "Show the whys and wherefores" .)