Just one not accepted by European shrink (5)
I believe the answer is:
equal
'just' is the definition.
(thesaurus)
'one not accepted by european shrink' is the wordplay.
'one' becomes 'i' (roman numeral).
'not accepted by' indicates named letters should be taken away.
'european' becomes 'e' (abbreviation e.g. EU).
'shrink' becomes 'quail' (synonyms).
'e'+'quail'='equail'
'equail' with 'i' removed is 'EQUAL'.
(Other definitions for equal that I've seen before include "Be as good as, match" , "The same in quantity, status etc" , "more or less" , "Commensurate" , "Match, rival" .)