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 see 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 "Unable to sit still", "Not relaxed, unsettled", "Agitated", "Fidgety - unable to sleep", "Uneasy, unable to relax", "Unable to relax or be still", "Worried and uneasy", "Ceaselessly in motion", "Uneasy and agitated" and "Fidgety, uneasy".)