A drink of whisky about the end of suppertime - it could disturb one's rest (5)
I believe the answer is:
dream
'it could disturb one's rest' is the definition.
The definition suggests a singular noun which matches the answer.
'a drink of whisky about the end of suppertime' is the wordplay.
'a drink of whisky' becomes 'dram' (I can't explain this - if you can you should believe this answer much more).
'about' is an insertion indicator.
'the end of' indicates one should take the final letters.
The final letter of 'suppertime' is 'e'.
'dram' going around 'e' is 'DREAM'.
Can you help me to learn more?
(Other definitions for dream that I've seen before include "King had one" , "Part of sleep" , "person to good to be true" , "Fancy (usually 7)" , "Perfect" .)