Sarcastic little girl in grass (8)
I believe the answer is:
scathing
'sarcastic' is the definition.
(I've seen this before)
'little girl in grass' is the wordplay.
'little girl' becomes 'cath' (I can't justify this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'grass' becomes 'sing' (I've seen this before).
'cath' going into 'sing' is 'SCATHING'.
Can you help me to learn more?
(Other definitions for scathing that I've seen before include "Caustic" , "Painful" , "Sarcastic" , "Severely critical" , "Witheringly scornful" .)