Smashing fellow deemed mad is lying in grass (10)
I believe the answer is:
shattering
'smashing' is the definition.
(I know that smashing can be written as shattering)
'is lying in grass' is the wordplay.
'is lying' becomes 'hatter' (I can't explain 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).
'hatter' put inside 'sing' is 'SHATTERING'.
'fellow deemed mad' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for shattering that I've seen before include "Totally exhausting; breaking" , "very upsetting" , "Very shocking" , "being destructive" , "with devastating result" .)