Sailor turns up 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 turns up with another' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'turns up' is a reversal indicator.
'with another' becomes 'salt' (I can't justify this - if you can you should give a lot more credence to this answer).
'ab' backwards is 'ba'.
'ba'+'salt'='BASALT'
Can you help me to learn more?
(Other definitions for basalt that I've seen before include "rock from 2 [VOLCANO]" , "Type of rock is a blast, oddly" , "Dark fine-grained volcanic rock" , "Rock containing olivine" , "Black volcanic rock" .)