A British rugby union player's opening try is most unexpected (9)

'most unexpected' is the definition.
Both the definition and answer are superlative adjectives. Perhaps they are linked in a way I don't understand?

'a british rugby union player's opening try' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in BBC).
'rugby union' becomes 'RU' (abbreviation).
'opening' indicates taking the first letters.
'try' becomes 'test' (synonyms).
The first letter of 'players' is 'p'.
'a'+'b'+'ru'+'p'+'test'='ABRUPTEST'

'is' is the link.

Can you help me to learn more?