Girl under water in populous borough (8)
I believe the answer is:
brooklyn
'populous borough' is the definition.
'brooklyn' can be an answer for 'borough' (borough of New York City). I'm not sure about the 'populous' bit.
'girl under water in' is the wordplay.
'girl' becomes 'Lyn'.
'under' says to put letters next to each other.
'water in' becomes 'brook' (brook is a kind of water. I am not sure about the 'in' bit.).
'lyn' after 'brook' is 'BROOKLYN'.
Can you help me to learn more?
(Other definitions for brooklyn that I've seen before include "part of Long Island?" , "New York borough with famous bridge." , "District and bridge in New York" , "New York City borough" , "Big Apple borough" .)