Assist Cockney when round US city (2,4)
I believe the answer is:
el paso
'us city' is the definition.
(city in Texas)
'assist cockney when round' is the wordplay.
'assist cockney' becomes ''elp' ('help' - synonyms - said with the 'H' dropped - a cockney accent.).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'round' becomes 'o' (round can mean a round object).
'elp'+'as'+'o'='EL PASO'
(Other definitions for el paso that I've seen before include "West Texas town" , "Texan city" , "City of Texas on the Rio Grande; aslope (anag.)" , "city on Mexican border" , "US city" .)