Place to plant hit in stomach — it hurts to fight (6,3)
I believe the answer is:
Here is my best explanation:
'place to plant' is the definition.
(place where plants may be planted)
'hit in stomach it hurts to fight' is the wordplay.
'hit in stomach' becomes 'wind'.
'it hurts' becomes 'ow' (expression of pain).
'to fight' becomes 'box' (boxing is a kind of fighting).
(Other definitions for window box that I've seen before include "Plants here", "Plant container on sill", "Narrow container for plants" and "where flowers may be found".)