Poor - really rotten! (5,3)
I believe the answer is:
badly off
'poor' is the definition.
(I've seen this before)
'really rotten' is the wordplay.
'really' becomes 'badly' (I am not sure about this - if you are sure you should believe this answer much more).
'rotten' becomes 'off' (rotten food is 'off').
'badly'+'off'='BADLY-OFF'
Can you help me to learn more?
(Other definitions for badly off that I've seen before include "Poor" , "Hard up" , "Poverty-stricken" .)