Cow not left in lowland? (8)
I believe the answer is:
frighten
'cow' is the definition.
(I've seen this before)
'not left in lowland?' is the wordplay.
'not left' becomes 'right' ('right' is the opposite to 'left').
'in' is an insertion indicator.
'lowland?' becomes 'fen' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'right' placed inside 'fen' is 'FRIGHTEN'.
Can you help me to learn more?
(Other definitions for frighten that I've seen before include "Terrify" , "Put the wind up" , "Cause fear" , "Cow" , "Spook" .)