An old rope product (5)
I believe the answer is:
oakum
'rope product' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'an old' is the wordplay.
I cannot quite see how this works, but
'an' could be 'a' and 'a' is found in the answer.
'old' could be 'o' (abbreviation as in OT - Old Testament) and 'o' is found within the answer.
The remaining letters 'kum' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for oakum that I've seen before include "Tarred ropes teased out for caulking ships" , "Rope fibre for caulking wooden ships" , "Caulking material" , "What convicts used to pick" , "Fibre from unravelled old rope" .)