Seal I see beneath spar (6)
I believe the answer is:
mastic
'seal' is the definition.
The answer and definition can be both substances as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'i see beneath spar' is the wordplay.
'see' becomes 'C' (the word for the letter, according to Chambers).
'beneath' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'spar' becomes 'mast' (mast is a kind of spar).
'i'+'c'='ic'
'ic' put after 'mast' is 'MASTIC'.
Can you help me to learn more?
(Other definitions for mastic that I've seen before include "Mouldable cement" , "Substance applied to joints" , "Gum from trees" , "Putty-like cement used as adhesive or filler" , "Waterproof sealant" .)