Tribal group determined to seize power (4)
I believe the answer is:
sept
'tribal group' is the definition.
(sept can mean an Irish clan)
'determined to seize power' is the wordplay.
'determined' becomes 'set' (synonyms).
'to seize' indicates putting letters inside.
'power' becomes 'p' (symbol in physics).
'set' enclosing 'p' is 'SEPT'.
(Other definitions for sept that I've seen before include "Branch of Irish clan" , "Short month" , "Month (abbreviation); clan subdivision (in Ireland)" , "Part of clan" , "Irish family group" .)