Almost revere a trainee officer (7)
I believe the answer is:
admiral
'officer' is the definition.
(admiral is a kind of officer)
'almost revere a trainee' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'revere' becomes 'admire' (I've seen this before).
'trainee' becomes 'l' (as in an L-plate for a learner driver).
'admire' with its final letter removed is 'admir'.
'admir'+'a'+'l'='ADMIRAL'
(Other definitions for admiral that I've seen before include "High-ranking naval officer like Nelson" , "Chief commander of fleet" , "Highest naval rank" , "Senior officer; mad liar (anag.)" , "(White or red) butterfly" .)