Detective’s assistant with a beginning of tricky issue (6)
I believe the answer is:
watson
'detective's assistant' is the definition.
(Dr Watson, Sherlock Holmes' assistant)
'with a beginning of tricky issue' is the wordplay.
'with' becomes 'w' (abbreviation).
'beginning of' suggests taking the first letters.
'issue' becomes 'son' (issue can mean someone's offspring).
The first letter of 'tricky' is 't'.
'w'+'a'+'t'+'son'='WATSON'
(Other definitions for watson that I've seen before include "Companion to 15 ac" , "Holmes' assistant" , "Holmes' associate" , "Doctor colleague of Sherlock Holmes" , "Crick's fellow geneticist" .)