Big city, one on western half of Salt Lake (5)
I believe the answer is:
nyasa
'lake' is the definition.
(I've seen this before)
'big city one on western half of salt' is the wordplay.
'big city' becomes 'ny' (New York).
'one' becomes 'a'.
'on' says to put letters next to each other.
'western half of' indicates taking half.
'salt' halved is 'sa'.
'ny'+'a'+'sa'='NYASA'
(Other definitions for nyasa that I've seen before include "Former name of Lake Malawi" , "lake in Africa" , "US city as a" , "Malawi, formerly" .)