Rock sailor overturned, then another (6)

I believe the answer is:

basalt

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'rock' is the definition.
(basalt is a kind of rock)

'sailor overturned then another' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'overturned' says the letters should be written in reverse.
'then another' becomes 'salt' (I can't explain this - if you can 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 "Commonest type of solidified lava; dense, dark grey and fine-grained" , "Dark fine-grained volcanic rock" , "Rock containing olivine" , "Common volcanic rock" , "Type of rock is a blast, oddly" .)

I've seen this clue in The Times.

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