Old girl encounters love in a big city (6)
I believe the answer is:
manila
'a big city' is the definition.
'Manila' can be an answer for 'city' (Manila is an example). I am not sure about the 'a big' bit.
'old girl encounters love in' is the wordplay.
'old girl encounters' becomes 'maa' (I can't justify this - if you can you should believe this answer much more).
'love' becomes 'nil' (love and nil mean zero in cricket and football).
'in' means one lot of letters goes inside another.
'maa' enclosing 'nil' is 'MANILA'.
Can you help me to learn more?
(Other definitions for manila that I've seen before include "Brown paper and Philippine city" , "Strong brown paper originally made from hemp" , "Animal or capital in the Phillipines" , "Strong brown paper; cigar, cheroot" , "Animal found in Philippines" .)