Thinly dispersed (6)
I believe the answer is:
'thinly dispersed' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sparse that I've seen before include "Meagre; scarce" , "Thin on the ground, scarce" , "Infrequent" , "One's not at all thick" , "In short supply" .)