Timber, erm, material once common in joints for seafarers (5)
I believe the answer is:
oakum
'timber' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'erm material once common in joints for seafarers' is the wordplay.
I cannot really see how this works, but
'material' could be 'oak' (oak is a kind of material) and 'oak' is found within the answer.
The remaining letters 'um' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for oakum that I've seen before include "What convicts used to pick" , "Rope fibre for caulking wooden ships" , "Tarred rope" , "Caulking material" , "ropy bits" .)