Unaffected by silly mime outside United Nations (6)
I believe the answer is:
immune
'unaffected' is the definition.
(similar in meaning)
'silly mime outside united nations' is the wordplay.
'silly' indicates anagramming the letters.
'outside' means one lot of letters goes inside another.
'united nations' becomes 'UN' (abbreviation).
'mime' anagrammed gives 'imme'.
'imme' placed around 'un' is 'IMMUNE'.
'by' is the link.
(Other definitions for immune that I've seen before include "Not subject or liable to" , "Inoculated or exempt" , "Not liable to, protected from" , "Not susceptible to, say after inoculation" , "safe from a particular disease" .)