It doesn't have a bull ring! (7)
I believe the answer is:
cowbell
'it doesn't' is the definition.
The definition suggests a singular noun which matches the answer.
'a bull ring' is the wordplay.
'a bull' becomes 'cow' (I can't explain this - if you can you should believe this answer much more).
'ring' becomes 'bell' (both can mean to call on the phone).
'cow'+'bell'='COWBELL'
'have' acts as a link.
Can you help me to learn more?
(Other definitions for cowbell that I've seen before include "It's heard among Alpine herds" , "Percussion instrument" , "alarm for rural use?" , "produced neat chime" .)