Soldiers drinking unknown whisky (3)
I believe the answer is:
rye
'whisky' is the definition.
(rye is a kind of whisky)
'soldiers drinking unknown' is the wordplay.
'soldiers' becomes 're' (Royal Engineers**).
'drinking' indicates putting letters inside.
'unknown' becomes 'y' (algebra).
're' placed around 'y' is 'RYE'.
(Other definitions for rye that I've seen before include "American whisky" , "New York bread" , "Hardy annual cereal grass" , "Type of whisky" , "Grain - whiskey - port" .)