Plan talk about source of Rhine (5)
I believe the answer is:
chart
'plan' is the definition.
(I've seen this before)
'talk about source of rhine' is the wordplay.
'talk' becomes 'chat' (I've seen this before).
'about' is an insertion indicator.
'source of' suggests taking the first letters.
The initial letter of 'rhine' is 'r'.
'chat' going around 'r' is 'CHART'.
(Other definitions for chart that I've seen before include "what could be a pie" , "[LOVE ME DO] became the first of many to ..." , "series of plots?" , "Visual display of information" , "Navigational map" .)