Onset of distinguished setter's swift decline (4)
I believe the answer is:
dive
'swift decline' is the definition.
(a sudden move downwards)
'onset of distinguished setter's' is the wordplay.
'onset of' indicates taking the first letters.
'setter's' becomes 'I've' ('I've', referring to the crossword setter).
The initial letter of 'distinguished' is 'd'.
'd'+'ive'='DIVE'
(Other definitions for dive that I've seen before include "Plunge, taken by some footballers" , "Plunge into - disreputable club" , "Disreputable bar; plunge" , "Headlong plunge into water" , "Plunge; low bar" .)