Run away from convert, spy and detective
I believe the answer is:
turn tail
'run away from' is the definition.
(thesaurus)
'convert spy and detective' is the wordplay.
'convert spy' becomes 'turn' (I can't justify this - if you can you should give a lot more credence to this answer).
'and' says to put letters next to each other.
'detective' becomes 'tail' (tail is a kind of detective).
'turn'+'tail'='TURN TAIL'
Can you help me to learn more?
(Other definitions for turn tail that I've seen before include "Prepare to flee" , "Cut and run" , "Spin round and run off" , "retreat" , "Take to one's heels" .)