Those written about rivers? (5)
I believe the answer is:
poems
'those' is the definition.
I can't judge whether this defines the answer.
'written about rivers?' is the wordplay.
'written' becomes 'em' (I am not sure about this - if you are sure you should believe this answer much more).
'about' is an insertion indicator.
'rivers?' becomes 'pos' (Po is an example).
'em' placed inside 'pos' is 'POEMS'.
Can you help me to learn more?
(Other definitions for poems that I've seen before include "Compositions, in metrical feet usually" , "Mopes about with verses" .)