Lack of a taxi upset philosopher (5)
I believe the answer is:
bacon
'philosopher' is the definition.
(Francis Bacon)
'lack of a taxi upset' is the wordplay.
'lack of' becomes 'no' (lack of money is no money).
'a taxi' becomes 'cab' ('cab' can be a synonym of 'taxi').
'upset' says the letters should be written backwards.
'no'+'cab'='nocab'
'nocab' backwards is 'BACON'.
(Other definitions for bacon that I've seen before include "Processed pig-meat" , "Shakespeare's ghost, some say" , "Breakfast meat?" , "Thin slices of pig" , "early scientist" .)