You find such hills as you walk around Middlesex (5)
I believe the answer is:
steep
'you find such hills as you' is the definition.
I can't tell whether this definition defines the answer.
'walk around middlesex' is the wordplay.
'walk' becomes 'step' (I've seen this before).
'around' means one lot of letters goes inside another.
'middlesex' becomes 'e' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'step' placed around 'e' is 'STEEP'.
Can you help me to learn more?
(Other definitions for steep that I've seen before include "Unreasonably dear - soak" , "Sharply inclined" , "Almost perpendicular" , "Difficult" , "Marinate" .)