Spread is less rapid when treated (9)
I believe the answer is:
dispersal
'spread' is the definition.
(I know that dispersal is a type of spread)
'less rapid when treated' is the wordplay.
'when treated' indicates anagramming the letters.
'less'+'rapid'='lessrapid'
'lessrapid' with letters rearranged gives 'DISPERSAL'.
'is' acts as a link.
(Other definitions for dispersal that I've seen before include "Scattering" , "Natural spread of seed" , "broadcast" , "Dissemination" , "(Of smoke) thinning out and disappearance" .)