Reproduce support on a fence opening (9)
I believe the answer is:
propagate
'reproduce' is the definition.
(I know that propagate is a type of reproduce)
'support on a fence opening' is the wordplay.
'support' becomes 'prop' (prop up can mean to support).
'on' is a charade indicator (letters next to each other).
'fence opening' becomes 'gate' (I've seen this in another clue).
'prop'+'a'+'gate'='PROPAGATE'
(Other definitions for propagate that I've seen before include "Disseminate" , "Generate by grafting or layering" , "Produce more" , "Publicise" , "broadcast" .)