Britain left European Union, turning to the right (4)
I believe the answer is:
blue
'right' is the definition.
(colour of the right-wing Conservative Party)
'britain left european union turning to' is the wordplay.
'britain' becomes 'B' (common abbreviation as in GB).
'left' becomes 'l' (common abbreviation).
'european' becomes 'e' (abbreviation e.g. EU).
'union' becomes 'U' (common abbreviation as in Trades Union Congress).
'turning to' is an anagram indicator (the letters can turn into other words).
'e'+'u'='eu'
'eu' with letters rearranged gives 'ue'.
'b'+'l'+'ue'='BLUE'
'the' is the link.
(Other definitions for blue that I've seen before include "Dispirited in cerulean way" , "Varsity sportsman" , "Azure" , "Red-headed Australian" , "Unsuitable for young" .)