Slough registers what John needs (3,5)

'what john needs' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'slough registers' is the wordplay.
I cannot really see how this works, but
'slough' could be 'bog' (I've seen this before) and 'bog' is found in the answer.
'registers' could be 'rolls' (lists of names) and 'rolls' is located in the answer.
No letters remain.
This explanation may well be incorrect...

Can you help me to learn more?