Final part in lower back to deteriorate (3)
I believe the answer is:
rot
'deteriorate' is the definition.
(I know that rot is a type of deteriorate)
'final part in lower back to' is the wordplay.
'final part in' indicates one should take the final letters (I've seen 'final parts in' mean this).
'back' says the letters should be written backwards.
The last letter of 'lower' is 'r'.
'to' reversed gives 'ot'.
'r'+'ot'='ROT'
(Other definitions for rot that I've seen before include "Decay in grotesque way" , "Codswallop" , "Informal nonsense" , "Corruption" , "Decay, perish" .)