Initially heard aboard railway - sources of the heavy metal beat (6)
I believe the answer is:
rhythm
'beat' is the definition.
(I know that beat can be written as rhythm)
'initially heard aboard railway sources of the heavy metal' is the wordplay.
'initially' suggests taking the first letters.
'aboard' is an insertion indicator.
'railway' becomes 'ry' (abbreviation for railway).
'sources of' suggests taking the first letters (the beginning of a word is its sauce).
The first letter of 'heard' is 'h'.
The first letters of 'the heavy metal' is 'thm'.
'h' put within 'ry' is 'rhy'.
'rhy'+'thm'='RHYTHM'
(Other definitions for rhythm that I've seen before include "'I Got - - ' (Gershwin)" , "Cycle -- cadence" , "Pattern of movement or sound" , "Strong regular repeated pattern of sound" , "Beat - time - metre" .)