Penniless pair keeping two ducks (4)
I believe the answer is:
poor
'penniless' is the definition.
(similar in meaning)
'pair keeping two ducks' is the wordplay.
'pair' becomes 'pr' (abbreviation).
'keeping' means one lot of letters goes inside another (some letters kept inside).
'two ducks' becomes 'oo'.
'pr' enclosing 'oo' is 'POOR'.
(Other definitions for poor that I've seen before include "Lacking money and resources" , "Indigent or substandard" , "Lacking means; paltry" , "'It's a . . . . heart that never rejoices'" , "Pants" .)