Sailor turning 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 turning up with another' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'turning up' is a reversal indicator.
'with another' becomes 'salt' (I can't explain this - if you can you should believe this answer much more).
'ab' written backwards gives 'ba'.
'ba'+'salt'='BASALT'
Can you help me to learn more?
(Other definitions for basalt that I've seen before include "Common volcanic rock" , "Commonest type of solidified lava; dense, dark grey and fine-grained" , "rock from 2 [VOLCANO]" , "Dark fine-grained volcanic rock" , "Dark rock of volcanic origin" .)