Explain that you can't see all the fish in the river (6)
I believe the answer is:
define
'explain that you can't see all' is the definition.
'define' can be an answer for 'explain' (I've seen this before). I'm not sure about the rest of the definition.
'the fish in the river' is the wordplay.
'the fish' becomes 'fin' (I can't justify this - if you can you should believe this answer much more).
'in the' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'fin' put within 'dee' is 'DEFINE'.
Can you help me to learn more?
(Other definitions for define that I've seen before include "Describe exactly" , "Explain the meaning of a word" , "Describe accurately" , "Specify" , "Fix the meaning of" .)