Organisation carried out investigation for Mariana Trench, for example, for the most part (10)
I believe the answer is:
unexplored
'the most part' is the definition.
Both the definition and answer are adjectives. Perhaps you can see a link between them that I don't see?
'organisation carried out investigation for mariana trench for example' is the wordplay.
I cannot really see how this works, but
'organisation' could be 'un' (UN is an example) and 'un' is found in the answer.
The remaining letters 'explored' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'for' is the link.
Can you help me to learn more?
(Another definition for unexplored that I've seen is " Not mapped".)