Opportunity to belt planter (6,3)
I believe the answer is:
window box
'planter' is the definition.
(I've seen this in another clue)
'opportunity to belt' is the wordplay.
'opportunity' becomes 'window' (I've seen this before).
'to belt' becomes 'box' (I can't justify this - if you can you should believe this answer much more).
'window'+'box'='WINDOW-BOX'
Can you help me to learn more?
(Other definitions for window box that I've seen before include "Plant container on sill" , "where flowers may be found" , "Narrow container for plants" .)