Fruit insect in grassland (8)
I believe the answer is:
plantain
'fruit' is the definition.
(I've seen this before)
'insect in grassland' is the wordplay.
'insect' becomes 'ant' (ant is a kind of insect).
'in' means one lot of letters goes inside another.
'grassland' becomes 'plain' (I have seen 'Open grassland' mean 'plain' so perhaps 'grassland' could also mean 'plain').
'ant' inserted within 'plain' is 'PLANTAIN'.
(Other definitions for plantain that I've seen before include "Shrub; green banana" , "Tropical food" , "Green-skinned banana" , "Banana-like fruit" , "Coarse banana" .)