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.
'pole' becomes 'n' (abbreviation for North, as in North Pole).
'to be in the' means one lot of letters goes inside another.
'kentish area' becomes 'SE' (South East of England).
'a'+'n'='an'
'an' put within 'se' is 'SANE'.
'for' is the link.
(Other definitions for sane that I've seen before include "Healthy in mind" , "Right-minded" , "Not mad" , "Rationable, sensible" , "Compos mentis" .)