Sensational Rugby Union back in top (5)
I believe the answer is:
lurid
'sensational' is the definition.
(similar in meaning)
'rugby union back in top' is the wordplay.
'rugby union' becomes 'RU'.
'back' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'top' becomes 'lid' (lid is a kind of top).
'ru' written backwards gives 'ur'.
'ur' inserted inside 'lid' is 'LURID'.
(Other definitions for lurid that I've seen before include "Ghastly bright" , "Shocking, sensational" , "Shockingly sensational" , "Melodramatically sensational" , "Grim" .)