Thought to have been regarded as (10)
I believe the answer is:
considered
'regarded as' is the definition.
'considered' can be an answer for 'regarded' ('consider' can be a synonym of 'regard'). I'm not sure about the 'as' bit.
'thought to have been' is the wordplay.
I cannot really understand how this works, but
'thought' could be 'side' (side is a kind of thought) and 'side' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for considered that I've seen before include "respected" , "Deliberated" , "Believed to be" , "Looked at" , "Thought carefully about" .)