Pull beneath the waves (8)
I believe the answer is:
undertow
'pull' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'beneath the waves' is the wordplay.
'beneath' becomes 'under' (under means beneath or underneath).
'the waves' becomes 'tow' (I can't explain this - if you can you should believe this answer much more).
'under'+'tow'='UNDERTOW'
Can you help me to learn more?
(Other definitions for undertow that I've seen before include "Submarine movement" , "Current, unforeseen" , "Current threat" , "New Tudor (anag.)" , "Current below surface fo sea" .)