Special forces are surrounding lake for a dip (5)
I believe the answer is:
salsa
'for a dip' is the definition.
(I've seen this before)
'special forces are surrounding lake' is the wordplay.
'special forces' becomes 'SAS' (Special Air Service).
'are' becomes 'a' (short for 'are', historical unit of measurement).
'surrounding' indicates putting letters inside.
'lake' becomes 'L' (geographical abbreviation).
'sas'+'a'='sasa'
'sasa' going around 'l' is 'SALSA'.
(Other definitions for salsa that I've seen before include "Tomato relish" , "Hot dance - hot sauce" , "Spicy tomato sauce" , "Dance or a dip?" , "Spicy 7" .)