American tribe ordered to capture republican judge (7)
I believe the answer is:
arbiter
'judge' is the definition.
(thesaurus)
'american tribe ordered to capture republican' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'ordered' indicates anagramming the letters.
'to capture' indicates putting letters inside.
'republican' becomes 'r' (abbreviation).
'tribe' anagrammed gives 'biter'.
'biter' going around 'r' is 'rbiter'.
'a'+'rbiter'='ARBITER'
(Other definitions for arbiter that I've seen before include "One has the authority to settle disputes" , "Rarebit? (anag)" , "He'll decide" , "Referee or judge of rarebit" , "One must exercise judgment" .)