He is a pound out on reflection (5)
I believe the answer is:
basil
'he' is the definition.
'is a pound out on reflection' is the wordplay.
'pound' becomes 'lb' (synonyms).
'out' means one lot of letters goes inside another.
'on reflection' says the letters should be written in reverse.
'is'+'a'='isa'
'isa' inserted inside 'lb' is 'lisab'.
'lisab' written backwards gives 'BASIL'.
(Other definitions for basil that I've seen before include "Herb - Mr Fawlty" , "Herb - boy's name" , "Culinary herb" , "Pesto ingredient" , "Aromatic leaves, used fresh or dried" .)