It's a setback when a trainee has to bow out (4)
I believe the answer is:
blow
'it's a setback' is the definition.
(blow can mean a setback or difficulty)
'a trainee has to bow out' is the wordplay.
I cannot quite understand how this works, but
'trainee' could be 'l' (as in an L-plate for a learner driver) and 'l' is located in the answer.
'to bow' could be 'bow' and 'bow' is found in the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
'when' acts as a link.
Can you help me to learn more?
(Other definitions for blow that I've seen before include "Shock; sound (a horn)" , "Blast" , "Breathe hard" , "Clip" , "Gust" .)