Nauseating thing picked up in City (6)
I believe the answer is:
emetic
'nauseating' is the definition.
(emetic means causing vomiting)
'thing picked up in city' is the wordplay.
'thing' becomes 'item' (I've seen this before).
'picked up' is a reversal indicator.
'in' means one lot of letters goes inside another.
'city' becomes 'EC' (the City of London postcode).
'item' back-to-front is 'meti'.
'meti' going within 'ec' is 'EMETIC'.
(Other definitions for emetic that I've seen before include "it will make one sick" , "Medicine used to cause vomiting" , "Ipecacuanha is one" , "Vomiting inducer" , "medicament" .)