Large plane happy to circle Californian airport (6)
I believe the answer is:
galaxy
'californian airport' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'large plane happy to circle' is the wordplay.
'large plane' becomes 'lax' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'happy' becomes 'gay' ('gay' can be similar in meaning to 'happy').
'to circle' is an insertion indicator.
'lax' placed inside 'gay' is 'GALAXY'.
Can you help me to learn more?
(Other definitions for galaxy that I've seen before include "The Milky Way, for example" , "Assemblage of stars" , "many stars" , "Splendid assemblage" , "Huge collection of stars" .)