Group of soldiers from the United Nations coming to city centre (4)
I believe the answer is:
unit
'group of soldiers' is the definition.
(unit can mean a group of soldiers)
'united nations coming to city centre' is the wordplay.
'united nations' becomes 'UN' (abbreviation).
'coming to' means one lot of letters go next to another.
'city centre' becomes 'it' (centre of the word 'city').
'un'+'it'='UNIT'
'from the' acts as a link.
(Other definitions for unit that I've seen before include "Single person - group of people" , "Quantity chosen as standard, or individual thing" , "Self-contained section" , "Individual component" , "army group" .)