It's reasonable for a Pole to be in the Kentish area (4)
I believe the answer is:
sane
'it's reasonable' is the definition.
(similar in meaning)
'a pole to be in the kentish area' is the wordplay.
'a pole' becomes 's' (abbreviation for South, as in South Pole).
'to be in the kentish area' becomes 'ane' (I can't explain this - if you can you should give a lot more credence to this answer).
's'+'ane'='SANE'
'for' is the link.
Can you help me to learn more?
(Other definitions for sane that I've seen before include "Healthy in mind" , "Right-minded" , "Not mad" , "Rationable, sensible" , "Compos mentis" .)