Tribe besieging city produces trumpet (7)
I believe the answer is:
clarion
'trumpet' is the definition.
(thesaurus)
'tribe besieging city' is the wordplay.
'tribe' becomes 'clan' ('clan' can be a synonym of 'tribe').
'besieging' is an insertion indicator.
'city' becomes 'rio' (Rio is an example**).
'clan' going around 'rio' is 'CLARION'.
'produces' is the link.
(Other definitions for clarion that I've seen before include "Loud, clear (call); old trumpet" , "Medieval brass instrument with a clear shrill tone" , "Medieval brass instrument heard in carol" , "Clear and ringing" , "Loud, clear and rousing" .)