Explain when bit of fish found in river (6)
I believe the answer is:
define
'explain' is the definition.
(thesaurus)
'bit of fish found in river' is the wordplay.
'bit of fish' becomes 'fin' ('fin' is part of 'fish').
'found in' means one lot of letters goes inside another.
'river' becomes 'dee' (River Dee).
'fin' going within 'dee' is 'DEFINE'.
'when' is the link.
(Other definitions for define that I've seen before include "State the meaning of" , "Give exact meaning as of a word" , "Specify" , "Explain the meaning of a word" , "Fix the meaning of" .)