One turns a bit boring (5)
I believe the answer is:
brace
'a bit boring' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'one turns' is the wordplay.
I cannot quite see how this works, but
'one' could be 'ace' (ace has a value of one in some card games) and 'ace' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for brace that I've seen before include "Stanchion" , "Stiffen; pair" , "Pair - supporting beam" , "A pair as of game birds" , "Support - pair" .)