Slip made by a proportion entering Swiss city (10)
I believe the answer is:
aberration
'slip' is the definition.
(thesaurus)
'a proportion entering swiss city' is the wordplay.
'proportion' becomes 'ratio' (I've seen this before).
'entering' means one lot of letters goes inside another.
'swiss city' becomes 'bern'.
'ratio' put within 'bern' is 'berration'.
'a'+'berration'='ABERRATION'
'made by' acts as a link.
(Other definitions for aberration that I've seen before include "Mental lapse" , "departure from normal" , "Deviation from the accepted norm" , "Deviation from normality" , "moral lapse" .)