Stop port being besieged by a tribe (7)
I believe the answer is:
clarion
'stop' is the definition.
(I've seen this before)
'port being besieged by a tribe' is the wordplay.
'port' becomes 'rio' (I've seen this before).
'being besieged by' is an insertion indicator (I've seen this in other clues).
'a tribe' becomes 'clan' ('clan' can be a synonym of 'tribe').
'rio' inserted within 'clan' is 'CLARION'.
(Other definitions for clarion that I've seen before include "In church maybe stop" , "Kind of trumpet" , "Loud, clear (call); old trumpet" , "Medieval brass instrument with a clear shrill tone" , "It makes a shrill note" .)