Those written about rivers? (5)
I believe the answer is:
poems
'those' is the definition.
I can't tell whether this definition defines the answer.
'written about rivers?' is the wordplay.
'written' becomes 'em' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' is an insertion indicator.
'rivers?' becomes 'pos' (Po is an example).
'em' inserted 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" .)