Ventured right inside the earth (5)

'earth' is the definition.
Both the definition and answer are singular nouns.
Perhaps there's an association between them I don't understand?

'ventured right inside the' is the wordplay.
'right' says to take the final letters (the letter on the right of the word).
'inside' becomes 'urs' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
The last letter of 'ventured' is 'd'.
'd'+'urs'+'t'='DURST'

Can you help me to learn more?