Misery of endless want keeping the old man in (7)
I believe the answer is:
despair
'misery' is the definition.
(I've seen this before)
'of endless want keeping the old man in' is the wordplay.
'of endless' means to remove the last letter (I've seen 'endless' mean this).
'want' becomes 'desire' (synonyms).
'keeping' indicates putting letters inside.
'the old man in' becomes 'pa' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'desire' with its last letter removed is 'desir'.
'desir' placed around 'pa' is 'DESPAIR'.
(Other definitions for despair that I've seen before include "Resignation" , "gloom" , "Feeling of hopelessness" , "Lose or lack hope" , "Misery" .)