The earth's five main features (6)
I believe the answer is:
oceans
'the earth's five main features' is the definition.
I can't judge whether this defines the answer.
'main features' is the wordplay.
'main' becomes 'ocean' (synonyms).
'features' becomes 's' (I can't justify this - if you can you should give a lot more credence to this answer).
'ocean'+'s'='OCEANS'
Can you help me to learn more?
(Other definitions for oceans that I've seen before include "Great seas" , "Atlantic, Pacific etc" , "Arctic and Indian" , "Large bodies of water" , "Vast bodies of water" .)