Mother's nervous complaint relates to varnish (6)
I believe the answer is:
mastic
'relates to varnish' is the definition.
I can't tell whether this defines the answer.
'mother's nervous complaint' is the wordplay.
'mother's' becomes 'ma's' (ma can mean mother).
'nervous complaint' becomes 'tic' (a tic is a medical complaint. I am not sure about the 'nervous' bit.).
'mas'+'tic'='MASTIC'
Can you help me to learn more?
(Other definitions for mastic that I've seen before include "Putty-like cement used as adhesive or filler" , "liquor" , "Waterproof sealant" , "Gum from trees" , "Mouldable cement" .)