Ship with answer for each Earth slave (6)
I believe the answer is:
vassal
'slave' is the definition.
(I've seen this before)
'ship with answer for each earth' is the wordplay.
I cannot quite see how this works, but
'ship' could be 'ss' (prefix in ship names eg SS Great Britain) and 'ss' is present in the answer.
'answer' could be 'a' (abbreviation eg in Q and A) and 'a' is found within the answer.
The remaining letters 'val' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for vassal that I've seen before include "A medieval servant" , "One owing fealty" , "Subordinate landholder" , "Feudal inferior" , "Old retainer" .)