Rock band's leader: an old sailor? (6)
I believe the answer is:
basalt
'rock' is the definition.
(I know that basalt is a type of volcanic rock)
'band's leader an old sailor?' is the wordplay.
'leader' says to take the initial letters.
'an' becomes 'a'.
'old sailor?' becomes 'salt' (old-fashioned nickname for a sailor).
The first letter of 'bands' is 'b'.
'b'+'a'+'salt'='BASALT'
(Other definitions for basalt that I've seen before include "Commonest type of solidified lava; dense, dark grey and fine-grained" , "Black volcanic rock" , "rock from 2 [VOLCANO]" , "Dark fine-grained volcanic rock" , "Common volcanic rock" .)