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' is a charade indicator (letters next to each other).
'city centre' becomes 'it' (centre of the word 'city').
'un'+'it'='UNIT'
'from the' is the 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" .)