Investigator longed to be even-handed (8)
I believe the answer is:
detached
'even-handed' is the definition.
Both the answer and definition are adjectives.
Perhaps there's an association between them I don't understand?
'investigator longed' is the wordplay.
'investigator' becomes 'det' (I can't justify this - if you can you should believe this answer much more).
'longed' becomes 'ached' ('ache' can be a synonym of 'long').
'det'+'ached'='DETACHED'
'to be' is the link.
Can you help me to learn more?
(Other definitions for detached that I've seen before include "Separate; aloof" , "Clinical" , "being sent on special assignment" , "cut off" , "Separated from - maybe a house" .)