Shot, dram occasionally, that’s often mixed with whisky (4)
I believe the answer is:
soda
'often mixed with whisky' is the definition.
I can't judge whether this definition defines the answer.
'shot dram occasionally' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'shot'+'dram'='shotdram'
The alternating letters of 'shotdram' are 'SODA'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for soda that I've seen before include "Variety of pop" , ""Splashed" drinks mixer" , "Sodium carbonate" , "Type of bread, water or fountain" , "Fizzy water, goes well with whiskey" .)