Brewer's agent definitely saves a ton (5)
I believe the answer is:
yeast
'brewer's agent' is the definition.
(chemical agent used when brewing alcohol)
'definitely saves a ton' is the wordplay.
'definitely' becomes 'yes' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'saves' indicates putting letters inside.
'ton' becomes 't'.
'yes' enclosing 'a' is 'yeas'.
'yeas'+'t'='YEAST'
Can you help me to learn more?
(Other definitions for yeast that I've seen before include "Agent for making bread" , "fermenter" , "It makes dough rise" , "Fungus able to convert sugar into alcohol" , "Fungus used in brewing and baking" .)