One, say, about to pull in line on river (6)
I believe the answer is:
angler
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'about to pull in line on river' is the wordplay.
'about' is an insertion indicator.
'to pull in' becomes 'ange' (I can't explain this - if you can you should believe this answer much more).
'line' becomes 'l' (used when specifying particular lines from a poem).
'on' says to put letters next to each other.
'river' becomes 'r' (abbreviation for river).
'ange' placed around 'l' is 'angle'.
'angle'+'r'='ANGLER'
'say' acts as a link.
Can you help me to learn more?
(Other definitions for angler that I've seen before include "Hook and line fisherman" , "A rod and line fisherman" , "Walton's was Compleat" , "One hoping for a catch" , "who looks for compliments" .)