Wet sauce, about two gallons (5)
I believe the answer is:
soggy
'wet' is the definition.
(similar in meaning)
'sauce about two gallons' is the wordplay.
'two gallons' means two substitutions for 'gallon'.
'sauce' becomes 'soy' (soy sauce).
'about' indicates putting letters inside.
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'g'+'g'='gg'
'soy' going around 'gg' is 'SOGGY'.
(Other definitions for soggy that I've seen before include "Squelchy" , "Saturated" , "Soft or heavy with moisture" , "rather wet" , "Heavy and moist with liquid" .)