Substance found in mass on Dart (6)
I believe the answer is:
marrow
'substance' is the definition.
(marrow is a kind of substance)
'mass on dart' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'on' means one lot of letters go next to another.
'dart' becomes 'arrow' (I've seen this before).
'm'+'arrow'='MARROW'
'found in' is the link.
(Other definitions for marrow that I've seen before include "Large courgette" , "one of two" , "Fatty stuff in bones, or a vegetable gourd" , "Large green squash" , "Stuff found inside bones" .)