Bad back thus ultimately hurts (7)
I believe the answer is:
offends
'hurts' is the definition.
('offend' can be a synonym of 'hurt')
'bad back thus ultimately' is the wordplay.
'bad' becomes 'off' (eg meat that's gone off is bad).
'back' becomes 'end' (I've seen this in other clues).
'ultimately' indicates one should take the final letters.
The final letter of 'thus' is 's'.
'off'+'end'+'s'='OFFENDS'
(Other definitions for offends that I've seen before include "Insults or causes displeasure to" , "annoys" , "Breaks the law" , "Hurts the feelings" , "Causes resentment" .)