Put off the fixed amount, about ten, for decline (13)
I believe the answer is:
deterioration
'decline' is the definition.
(I know that deterioration is a type of decline)
'put off the fixed amount about ten' is the wordplay.
'put off' becomes 'deter' (both can mean to discourage).
'the fixed amount' becomes 'ration'.
'about' is a charade indicator (letters next to each other).
'ten' becomes 'IO' (IO resembles 10).
'ration' after 'io' is 'ioration'.
'deter'+'ioration'='DETERIORATION'
'for' acts as a link.
(Other definitions for deterioration that I've seen before include "Rot!" , "Process of worsening" , "A progressive worsening" .)