Kind of varnish on the mast, I see (6)
I believe the answer is:
mastic
'kind of varnish' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'on the mast i see' is the wordplay.
'on' means one lot of letters go next to another.
'see' becomes 'C' (the word for the letter, according to Chambers).
'mast'+'i' is 'masti'.
'masti'+'c'='MASTIC'
Can you help me to learn more?
(Other definitions for mastic that I've seen before include "Waterproof sealant" , "Resin used in varnishes" , "Gum from trees" , "Mouldable cement" , "Aromatic resin used to make varnish" .)