Trainee ultimately failing to pass (6)
I believe the answer is:
elapse
'pass' is the definition.
(I've seen this before)
'trainee ultimately failing' is the wordplay.
'ultimately' says to take the final letters.
'failing' becomes 'lapse' (I've seen this before).
The last letter of 'trainee' is 'e'.
'e'+'lapse'='ELAPSE'
'to' acts as a link.
(Other definitions for elapse that I've seen before include "pass in silence" , "Go by - pass away" , "(Of time) go by" , "'Pass by, as time (6)'" , "Go past" .)