Weightwatchers finish with detailed brief (7)
I believe the answer is:
dieters
'weightwatchers' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'finish with detailed brief' is the wordplay.
'finish' becomes 'die'.
'with detailed brief' becomes 'ters' (I can't justify this - if you can you should give a lot more credence to this answer).
'die'+'ters'='DIETERS'
Can you help me to learn more?
(Another definition for dieters that I've seen is " people trying to lose weight".)