18 across
Scotch:
old
fool
at
first
returned
one
litre
(4)
Scotch: old fool at first returned one litre (4)
hints
0
checks
0
hints
The definition is at the beginning of the clue.
How the clue works
Here Scotch is being used as a verb, meaning to thwart. Take O (old) and F (the first letter of "fool"), then reverse them to get FO, and tack on I (one) and L (litre) to finish the job.