Play with internal river chart that describes landscape (10)
I believe the answer is:
topography
'landscape' is the definition.
(I've seen this before)
'play with internal river chart' is the wordplay.
'play' becomes 'toy' (synonyms).
'with internal' indicates putting letters inside.
'river' becomes 'Po' (Italian river).
'chart' becomes 'graph' (I've seen this before).
'po'+'graph'='pograph'
'toy' enclosing 'pograph' is 'TOPOGRAPHY'.
'that describes' is the link.
(Other definitions for topography that I've seen before include "landscape" , "contents of map" , "Surface features of a region" .)