Relative managed to break into key US city (6)
I believe the answer is:
granny
'relative' is the definition.
(I know that granny is a type of relative)
'managed to break into key us city' is the wordplay.
'managed' becomes 'ran' (both can mean organised or was in charge of).
'to break into' means one lot of letters goes inside another (inserted letters break into the word).
'key' becomes 'g' (musical key).
'us city' becomes 'NY' (New York).
'g'+'ny'='gny'
'ran' put into 'gny' is 'GRANNY'.
(Other definitions for granny that I've seen before include "A parent's mother" , "Elder relative" , "Insecure knot" , "one possibly given a flat?" , "Female relative" .)