Visible evidence of past civilisation is yet to be discovered (7,2,2,4)
I believe the answer is:
remains to be seen
'visible evidence of past civilisation' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'is yet to be discovered' is the wordplay.
I cannot quite see how this works, but
'is' could be 'be' and 'be' is found in the answer.
'to' is present in the answer.
'be' could be 'remain' (remaining is a kind of being) and 'remain' is found within the answer.
'discovered' could be 'seen' (I've seen this in another clue) and 'seen' is present in the answer.
A single letter 's' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?