Even now oceans are difficult to fathom, we're told (5,6,3,4)
I believe the answer is:
still waters run deep
'we're told' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'even now oceans are difficult to fathom' is the wordplay.
I cannot really see how this works, but
'even' could be 'still' (synonyms) and 'still' is present in the answer.
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is found within the answer.
'difficult to fathom' could be 'deep' (I've seen this in another clue) and 'deep' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for still waters run deep that I've seen before include "Yet there are undercurrents" , "Quiet demeanour" , "Quiet people are full of (dangerous) ideas" , "more than you suspect, I think" , "may give no hint of what happens when one gets to bed" .)