Tries once more before end of the drill (8)
I believe the answer is:
rehearse
'drill' is the definition.
(I've seen this in another clue)
'tries once more before end of the' is the wordplay.
'tries once more' becomes 'rehears' ('hears' can be a synonym for 'tries' so perhaps 'rehears' is OK for 'tries once more').
'before' says to put letters next to each other.
'end of' suggests the final letters.
The final letter of 'the' is 'e'.
'rehears'+'e'='REHEARSE'
(Other definitions for rehearse that I've seen before include "Prepare for a stage production" , "Train" , "Practise parts" , "What actor does to prepare for play" , "prepare for performance" .)