Before start of event, tries again to practise (8)
I believe the answer is:
rehearse
'to practise' is the definition.
(synonyms)
'before start of event tries again' is the wordplay.
'before' says to put letters next to each other.
'start of' suggests taking the first letters.
'tries again' becomes 'rehears' ('hears' can be a synonym for 'tries' so perhaps 'rehears' is OK for 'tries again').
The first letter of 'event' is 'e'.
'e' put after 'rehears' is 'REHEARSE'.
(Other definitions for rehearse that I've seen before include "Train" , "Practise - run through" , "Practise (one's lines)" , "Run through (a play)" , "Recite - try out" .)