Thinker secures it in UK (7)
I believe the answer is:
britain
'uk' is the definition.
(synonyms)
'thinker secures it' is the wordplay.
'thinker' becomes 'brain' (I've seen this in another clue).
'secures' means one lot of letters goes inside another.
'brain' enclosing 'it' is 'BRITAIN'.
'in' acts as a link.
(Other definitions for britain that I've seen before include "Country" , "State" , "Where we live - Great!" , "England, Scotland, Wales and Northern Ireland" , "See 10" .)