Back to revere, almost, a trainee officer (4,7)
I believe the answer is:
rear admiral
'officer' is the definition.
(I know that rear admiral is a type of officer)
'back to revere almost a trainee' is the wordplay.
'back' becomes 'rear' (something's rear is its back).
'to revere' becomes 'admire' (I've seen this before).
'almost' means to remove the last letter (most of the word but not all of it).
'a trainee' becomes 'al' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'admire' with its last letter taken off is 'admir'.
'rear'+'admir'+'al'='REAR-ADMIRAL'
Can you help me to learn more?
(Other definitions for rear admiral that I've seen before include "Naval rank above commodore" , "navy bigwig" , "Naval officer" , "Top brass" .)