Tribe in charge of eastern part of UK (5)
I believe the answer is:
iceni
'tribe' is the definition.
(I've seen this before)
'in charge of eastern part of uk' is the wordplay.
'in charge of' becomes 'ic'.
'eastern' becomes 'e'.
'part of uk' becomes 'ni' (Northern Ireland).
'ic'+'e'+'ni'='ICENI'
(Other definitions for iceni that I've seen before include "Ancient British tribe" , "Ancient Brits" , "people rebelling against Rome" , "Old warriors" , "British people once" .)