Weak spirit and old wine (8)
I believe the answer is:
impotent
'weak' is the definition.
(similar in meaning)
'spirit and old wine' is the wordplay.
'spirit' becomes 'imp' (I have seen 'Evil spirit ' mean 'imp' so perhaps 'spirit' could also mean 'imp').
'and' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'wine' becomes 'tent' (I've seen this before).
'imp'+'o'+'tent'='IMPOTENT'
(Other definitions for impotent that I've seen before include "Ineffective" , "Lacking power" , "Mon petit may be powerless" , "Helpless, powerless" , "Powerless like mon petit" .)