Decline to put off one formal speech (13)
I believe the answer is:
deterioration
'decline' is the definition.
(deterioration is a kind of decline)
'put off one formal speech' is the wordplay.
'put off' becomes 'deter' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'formal speech' becomes 'oration' (I've seen this before).
'deter'+'i'+'oration'='DETERIORATION'
'to' acts as a link.
(Other definitions for deterioration that I've seen before include "Rot!" , "A progressive worsening" , "Process of worsening" .)