Waterproof, this is instinctive: spill tea! (5-6)
I believe the answer is:
gutta-percha
'waterproof this' is the definition.
The definition suggests a singular noun which matches the answer.
'instinctive spill tea' is the wordplay.
'instinctive' becomes 'gut' (as in a gut feeling).
'spill' becomes 'taper' (I've seen this before).
'tea' becomes 'cha' (cha is a type of tea).
'gut'+'taper'+'cha'='GUTTA-PERCHA'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for gutta-percha that I've seen before include "Hard rubber-like latex" , "Rubber-like latex product" , "Rubber-like substance" , "Substance like hard rubber" .)