Penitent pruned the Spanish chestnut (6)
I believe the answer is:
sorrel
'chestnut' is the definition.
(I've seen this in another clue)
'penitent pruned the spanish' is the wordplay.
'penitent' becomes 'sorry' (similar in meaning).
'pruned' means to remove the last letter.
'the spanish' becomes 'el' ('the' in Spanish).
'sorry' with its last letter taken away is 'sorr'.
'sorr'+'el'='SORREL'
(Other definitions for sorrel that I've seen before include "Edible plant of the dock family" , "Light chestnut" , "Acid-tasting salad leaf" , "Reddish brown horse or sharp-tasting herb" , "Edible plant, horse of light reddish-brown coat" .)