Practise, seeing Shearer troubled before Newcastle's final (8)
I believe the answer is:
rehearse
'practise' is the definition.
('rehearse' can be a synonym of 'practise')
'shearer troubled before newcastle's final' is the wordplay.
'troubled' is an anagram indicator.
'before' says to put letters next to each other.
'final' says to take the final letters.
The last letter of 'newcastle' is 'e'.
'shearer' anagrammed gives 'rehears'.
'rehears'+'e'='REHEARSE'
'seeing' is the link.
(Other definitions for rehearse that I've seen before include "Run through (a play)" , "Train" , "Practise parts" , "Practise (one's lines)" , "Recite - try out" .)