Be very protective of girl in US city (5)
I believe the answer is:
nanny
'be very protective of' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'girl in us city' is the wordplay.
'girl' becomes 'ann' (woman's name).
'in' indicates putting letters inside.
'us city' becomes 'NY' (short for New York).
'ann' put into 'ny' is 'NANNY'.
Can you help me to learn more?
(Other definitions for nanny that I've seen before include "eg Mary Poppins" , "Home help" , "Undue government interference with personal choice?" , "Issue tender" , "Posh child minder or female goat" .)