Thought to be a water reed of a new kind (8)
I believe the answer is:
pondered
'thought' is the definition.
(pondering is a kind of thinking)
'a water reed of a new kind' is the wordplay.
I cannot really see how this works, but
'water' could be 'pond' (pond is a kind of water) and 'pond' is found in the answer.
an anagram of 'reed' is 'ered' which is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'to be' is the link.
Can you help me to learn more?
(Other definitions for pondered that I've seen before include "Reflected deeply" , "Cogitated" , "Contemplated deeply" , "Considered carefully" , "Gave thought to" .)