They swim and dive daily in groups of swimmers (8)
I believe the answer is:
pochards
'they swim and dive' is the definition.
The definition suggests an adverb but the answer is not.
'daily in groups of swimmers' is the wordplay.
'daily' becomes 'char' (both can mean a cleaner).
'in' means one lot of letters goes inside another.
'groups of swimmers' becomes 'pods' (I can't explain this - if you can you should give a lot more credence to this answer).
'char' inserted into 'pods' is 'POCHARDS'.
Can you help me to learn more?
(Other definitions for pochards that I've seen before include "Birds" , "ducks" .)