Old boy, fool, sent back soup (5)
I believe the answer is:
gumbo
'soup' is the definition.
(type of Cajun soup)
'old boy fool sent back' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'fool' becomes 'mug' (synonyms).
'sent back' says the letters should be written in reverse.
'ob'+'mug'='obmug'
'obmug' reversed gives 'GUMBO'.
(Other definitions for gumbo that I've seen before include "Thick soup" , "Okra; spicy Cajun dish" , "Spicy soup; okra" , "Okra pods" , "Cajun chicken dish" .)