Indifferent murmur to trainee (4)
I believe the answer is:
cool
'indifferent' is the definition.
(I've seen this in another clue)
'murmur to trainee' is the wordplay.
'murmur' becomes 'coo' (cooing is a kind of murmuring).
'to' means one lot of letters go next to another.
'trainee' becomes 'l' (as in an L-plate for a learner driver).
'coo'+'l'='COOL'
(Other definitions for cool that I've seen before include "Calm and collected" , "Sangfroid" , "Moderately cold" , "Chilled" , "Modern calmness, brrr!" .)