Just clever to abandon Britain (5)
I believe the answer is:
right
'just' is the definition.
(associated in meaning)
'clever to abandon britain' is the wordplay.
'clever' becomes 'bright' (I've seen this before).
'to abandon' indicates named letters should be taken away.
'britain' becomes 'b' (common abbreviation as in GB).
'bright' with 'b' taken away is 'RIGHT'.
(Other definitions for right that I've seen before include "Morally good, justifiable or acceptable" , "blue, politically" , "Spot-on" , "Valid" , "Punch" .)