Sailor turns up with another rock (6)
I believe the answer is:
basalt
'rock' is the definition.
(basalt is a kind of rock)
'sailor turns up with another' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'turns up' says the letters should be written backwards.
'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'
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" .)