Tries again with energy to practise (8)
I believe the answer is:
rehearse
'practise' is the definition.
('rehearse' can be a synonym of 'practise')
'tries again with energy' is the wordplay.
'tries again' becomes 'rehears' ('hears' can be a synonym for 'tries' so perhaps 'rehears' is OK for 'tries again').
'with' means one lot of letters go next to another.
'energy' becomes 'e' (physics symbol as in E=mc2).
'rehears'+'e'='REHEARSE'
'to' acts as a link.
(Other definitions for rehearse that I've seen before include "Prepare for a stage production" , "Practice a play" , "Practice for the performance" , "Train" , "Practise (one's lines)" .)