Breed to support a barrier (9)
I believe the answer is:
propagate
'breed' is the definition.
(I've seen this before)
'support a barrier' is the wordplay.
'support' becomes 'prop' (prop is a kind of support).
'barrier' becomes 'gate' (gate is a kind of barrier).
'prop'+'a'+'gate'='PROPAGATE'
'to' acts as a link.
(Other definitions for propagate that I've seen before include "Develop" , "broadcast" , "widely spread" , "Advertise widely" , "Produce more" .)