Rugby union back in top that’s garish (5)
I believe the answer is:
lurid
'that's garish' is the definition.
(I've seen this before)
'rugby union back in top' is the wordplay.
'rugby union' becomes 'RU'.
'back' is a reversal indicator.
'in' means one lot of letters goes inside another.
'top' becomes 'lid' (lid is a kind of top).
'ru' reversed gives 'ur'.
'ur' inserted within 'lid' is 'LURID'.
(Other definitions for lurid that I've seen before include "Horrifying - sensational" , "Sensational or dazzling" , "In vivid detail" , "Bright and garish" , "Shocking, sensational" .)