Sailor returns with another rock (6)
I believe the answer is:
basalt
'rock' is the definition.
(I know that basalt is a type of volcanic rock)
'sailor returns with another' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'returns' says the letters should be written in reverse.
'with another' becomes 'salt' (I am not sure about this - if you are sure you should believe this answer much more).
'ab' backwards is 'ba'.
'ba'+'salt'='BASALT'
(Other definitions for basalt that I've seen before include "Black volcanic rock" , "Type of rock is a blast, oddly" , "rock from 2 [VOLCANO]" , "Dark fine-grained volcanic rock" , "Common volcanic rock" .)