Relative of American set free (6)
I believe the answer is:
auntie
'relative' is the definition.
(auntie is a kind of relative)
'american set free' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'set free' becomes 'untie' (I've seen this before).
'a'+'untie'='AUNTIE'
'of' is the link.
(Other definitions for auntie that I've seen before include "Relation; the BBC" , "Female relative" , "woman in the family" , "Untie a relative" , "BBC; relative (colloq,)" .)