Bright blue rings stolen ultimately, sucker! (9)
I believe the answer is:
quicksand
'sucker' is the definition.
(something which sucks)
'bright blue rings stolen ultimately' is the wordplay.
'bright' becomes 'quick'.
'blue' becomes 'sad' ('sad' can be similar in meaning to 'blue').
'rings' is an insertion indicator.
'ultimately' suggests the final letters.
The final letter of 'stolen' is 'n'.
'sad' enclosing 'n' is 'sand'.
'quick'+'sand'='QUICKSAND'
(Other definitions for quicksand that I've seen before include "Area of loose wet beach where one may sink" , "Ground which sucks!" , "Treacherous terrain" , "Treacherous situation (which sucks people and things down)" , "Treacherous situation that can trap and destroy" .)