Eminent detective used insight at work (13)
I believe the answer is:
distinguished
'eminent' is the definition.
(synonyms)
'detective used insight at work' is the wordplay.
'detective' becomes 'di' (Detective Inspector abbreviation).
'at work' is an anagram indicator.
'used'+'insight'='usedinsight'
'usedinsight' is an anagram of 'stinguished'.
'di'+'stinguished'='DISTINGUISHED'
(Other definitions for distinguished that I've seen before include "saw the difference" , "Recognised - famous" , "Made out; noble" , "Acclaimed" , "Made out; illustrious" .)