Sun or Earth: one has to go! (3)
I believe the answer is:
sol
'sun' is the definition.
(Latin for 'sun')
'earth one has to go' is the wordplay.
'earth' becomes 'soil' (earth can mean soil or dirt).
'one' becomes 'i' (Roman numeral).
'has to go' suggests deleting specific letters.
'soil' with 'i' removed is 'SOL'.
'or' is the link.
(Other definitions for sol that I've seen before include "Roman counterpart of Helios" , "Note; Peruvian currency" , "Spanish and poetic sun" , "Fifth note of scale" , "Roman sun-god" .)