Deprived chap briefly kept by Napoleonic marshal (5)
I believe the answer is:
needy
'deprived' is the definition.
(I've seen this before)
'chap briefly kept by napoleonic marshal' is the wordplay.
'chap briefly' becomes 'Ed' (short for Edward; 'briefly' indicates an abbreviation).
'kept by' indicates putting letters inside.
'napoleonic marshal' becomes 'ney' (Michel Ney, French military commander).
'ed' put within 'ney' is 'NEEDY'.
(Other definitions for needy that I've seen before include "penniless" , "Poor; wanting affection" , "badly craving affection" , "Poor - destitute" , "Poverty-stricken" .)