Measure needing no introduction in the past for a back condition (7)
I believe the answer is:
lumbago
'a back condition' is the definition.
(lumbago is a condition affecting the lower back)
'measure needing no introduction in the past' is the wordplay.
'measure' becomes 'plumb' (to plumb a body of water is to measure its depth).
'needing no introduction' means to remove the first letter.
'in the past' becomes 'ago' (three years ago means three years in the past).
'plumb' with its first letter removed is 'lumb'.
'lumb'+'ago'='LUMBAGO'
'for' is the link.
(Other definitions for lumbago that I've seen before include "Rheumatic pain in muscles of lower back" , "Maul bog badly for a rheumatic affliction" , "Back complaint" , "Painful condition" , "Lower back ache" .)