Maybe insomniac struggles without whiskey and splash of soda? (8)
I believe the answer is:
Here is my best explanation:
'maybe insomniac' is the definition.
'struggles without whiskey and splash of soda?' is the subsidiary indication.
I cannot quite understand how this works, but
'without' could be 'less' and 'less' is present in the answer.
The remaining letters 'rest' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
(Other definitions for restless that I've seen before include "Uneasy and agitated", "Fidgety - unable to sleep", "Always on the move", "Ceaselessly in motion", "Uneasy, unable to relax", "Unable to relax or be still", "Fidgety, uneasy", "Worried and uneasy", "Fidgetty" and "Constantly moving about or fidgeting".)