Daughter, with bad back, in decline (5)
I believe the answer is:
droop
'decline' is the definition.
(I've seen this before)
'daughter with bad back' is the wordplay.
'daughter' becomes 'd' (genealogical abbreviation).
'with' is a charade indicator (letters next to each other).
'bad' becomes 'poor' (similar in meaning).
'back' says the letters should be written backwards.
'poor' back-to-front is 'roop'.
'd'+'roop'='DROOP'
'in' is the link.
(Other definitions for droop that I've seen before include "Sag wearily" , "Hang down loosely" , "lose heart" , "Flag" , "be despondent" .)