Mud on east of big road (4)
I believe the answer is:
mire
'mud' is the definition.
(I know that mud can be written as mire)
'on east of big road' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'east of' means one lot of letters go next to another (some letters go to the right of others).
'big road' becomes 'mi' (resembles M1 motorway. I am not sure about the 'big' bit.).
're' put after 'mi' is 'MIRE'.
Can you help me to learn more?
(Other definitions for mire that I've seen before include "Emir found in mud" , "Bog - muck" , "Deep soft mud" , "Cause to get stuck (in soft mud?)" , "Marsh" .)