Senior police officer, first-class (5)
I believe the answer is:
super
'first-class' is the definition.
(synonyms)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for super that I've seen before include "First-rate, top-notch" , "Topnotch, terrific" , "Marvellous, terrific" , "Peerless" , "Great; wonderful" .)