Clue: A sound measure of the third man (4)
Answer: ABEL
Explanation: Definition: the third man
A charade of 'a' plus BEL ('sound measure'; as it is a rather large physical unit, the tenth part, decibel, is much better known). We had ABEL as 'the third man' – 'man' as masculine, preceded by Adam and Cain – in Mondays' Rufus.