Member of police force becomes tense during bust (9)
I believe the answer is:
detective
'member of police' is the definition.
(type of police officer)
I don't understand how the rest of the clue works.
(Other definitions for detective that I've seen before include "Expert solver of clues" , "Criminal investigator" , "Public or private investigator" , "Private or police investigator" , "Policeman, say" .)