Examine one fire set for witness (9)
I believe the answer is:
testifier
'for witness' is the definition.
(testifier is a kind of witness)
'examine one fire set' is the wordplay.
'examine' becomes 'test' (testing is a kind of examining).
'one' becomes 'i' (Roman numeral).
'set' indicates an anagram (instruction to set the letters).
'fire' with letters rearranged gives 'fier'.
'test'+'i'+'fier'='TESTIFIER'
(Other definitions for testifier that I've seen before include "Witness" , "person in court?" , "One who may declare" , "One acknowledging" .)