Add last of ginger to hot liquid, that's immature! (5)
I believe the answer is:
larva
'that's immature' is the definition.
I don't understand the remainder of the clue.
(Other definitions for larva that I've seen before include "Caterpillar, for example" , "Tadpole or caterpillar, say" , "immature six-footer?" , "Volcanic outpouring" , "a developmental stage" .)