Such varnish seems a bit sticky to mum (6)
I believe the answer is:
mastic
'such varnish seems a bit' is the definition.
I can't judge whether this definition defines the answer.
'sticky to mum' is the wordplay.
'sticky' becomes 'stic' (I can't explain this - if you can you should give a lot more credence to this answer).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'mum' becomes 'ma' (both can mean mother).
'stic' put after 'ma' is 'MASTIC'.
Can you help me to learn more?
(Other definitions for mastic that I've seen before include "Aromatic resin used to make varnish" , "Mouldable cement" , "Resin used in varnishes" , "Gum from trees" , "Putty-like cement used as adhesive or filler" .)