Gives one a start, perhaps, but just in the Fens (9)
I believe the answer is:
frightens
'gives one a start perhaps' is the definition.
I can't judge whether this defines the answer.
'just in the fens' is the wordplay.
'just' becomes 'right' (similar in meaning).
'in' means one lot of letters goes inside another.
'right' inserted into 'fens' is 'FRIGHTENS'.
'but' acts as a link.
Can you help me to learn more?
(Other definitions for frightens that I've seen before include "Causes fear" , "Cows" , "Alarms, scares" .)