Small stem (7)
'small stem' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sprig that I've seen before include "Stem of holly, for example" , "piece of holly" , "Small leafy stem" , "Small nail without a head" , "Stem or spray" .)
(Other definitions for rootlet that I've seen before include "Sucker (Bot)" , "Small radical" , "underground development" , "Little plant part" , "Minor growth under a plant" .)