An opening for trainee soldier (3)
I believe the answer is:
ant
'soldier' is the definition.
(soldier ant is a type of ant)
'an opening for trainee' is the wordplay.
'opening for' says to take the initial letters.
The initial letter of 'trainee' is 't'.
'an'+'t'='ANT'
(Other definitions for ant that I've seen before include "One more provident than Grasshopper" , "Emmet or pismire" , "A soldier, for one" , "The insect lives in a social colony" , "Industrious insect" .)