Land fish, one that's out of this world? (9)
I believe the answer is:
earthling
'one that's out of this world?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'land fish' is the wordplay.
'land' becomes 'earth' ('earth' can be a synonym of 'land'**).
'fish' becomes 'ling' (ling is a kind of fish).
'earth'+'ling'='EARTHLING'
Can you help me to learn more?
(Other definitions for earthling that I've seen before include "Human being (sci-fi)" , "sci-fi character" , "man so to be called from space?" , "Man of the world" .)