In retrospect police unit say Del Boy's brief was originally an accessory (6,3)

'an accessory' is the definition.
I don't know anything about this answer so I can't tell whether this works.

'in retrospect police unit say del boy's brief was originally' is the wordplay.
'in retrospect' is a reversal indicator.
'police unit' becomes 'CID' (abbreviation for Criminal Investigation Department).
'say del' becomes 'key' (I can't justify this - if you can you should believe this answer much more).
'brief' means to remove the last letter (a shorter or briefer version of the word).
'originally' suggests taking the first letters.
The initial letter of 'was' is 'w'.
'cid' reversed gives 'dic'.
'boy' with its final letter removed is 'bo'.
'dic'+'key'+'bo'+'w'='DICKEY BOW'

Can you help me to learn more?