O'Sullivan's third trick shot enthrals the UK, for one (7)
I believe the answer is:
country
'uk for one' is the definition.
(uk is a kind of country)
'o'sullivan's third trick shot enthrals' is the wordplay.
'o'sullivan's third' becomes 'u' (I am not sure about this - if you are sure you should believe this answer much more).
'trick' becomes 'con' (a con is a fraud or trick).
'shot' becomes 'try' (both can mean an attempt).
'enthrals' is an insertion indicator.
'con'+'try'='contry'
'u' going within 'contry' is 'COUNTRY'.
'the' is the link.
Can you help me to learn more?
(Other definitions for country that I've seen before include "Rural" , "land?" , "See 9" , "State; region" , "Nation" .)