Extensive weeds with rapid dispersing (10)
I believe the answer is:
widespread
'extensive' is the definition.
(I've seen this before)
'weeds with rapid dispersing' is the wordplay.
'with' says to put letters next to each other.
'dispersing' indicates anagramming the letters.
'weeds'+'rapid'='weedsrapid'
'weedsrapid' anagrammed gives 'WIDESPREAD'.
(Other definitions for widespread that I've seen before include "common" , "broad" , "Generally prevalent" , "found in many places" , "Sweeping" .)