Fellow initially brought in by police investigator as expert (4)
I believe the answer is:
deft
'expert' is the definition.
(I've seen this in another clue)
'fellow initially brought in by police investigator' is the wordplay.
'initially' indicates taking the first letters.
'brought in by' means one lot of letters goes inside another.
'police investigator' becomes 'det' (I am not sure about this - if you are sure you should believe this answer much more).
The first letter of 'fellow' is 'f'.
'f' inserted into 'det' is 'DEFT'.
'as' is the link.
Can you help me to learn more?
(Other definitions for deft that I've seen before include "Dextrous" , "Adriot and skilful" , "Quick and skilful in movement" , "Adroit, dexterous" , "Handy" .)