A herb for Mr Fawlty (5)
I believe the answer is:
basil
I believe this is a triple definition!
'a herb' is the first definition.
(basil is a herb)
'for mr' is the second definition.
'fawlty' is the third definition.
(Basil Fawlty of the sitcom Fawlty Towers)
(Other definitions for basil that I've seen before include "Mediterranean herb" , "Pesto ingredient" , "Bails out the herb" , "Fellow" , "Herb - boy's name" .)