Unknown round surface that alien guards? (9)
I believe the answer is:
exoplanet
'guards?' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'unknown round surface that alien' is the wordplay.
I cannot really see how this works, but
'unknown' could be 'x' (algebra) and 'x' is found in the answer.
'round' could be 'o' (o is a circular letter) and 'o' is found within the answer.
'surface' could be 'plane' (I've seen this before) and 'plane' is present in the answer.
'alien' could be 'et' (extra terrestrial) and 'et' is found within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for exoplanet that I've seen before include "where no human has ever been" , "distant world" .)