Largely respect a trainee naval officer (7)
I believe the answer is:
admiral
'naval officer' is the definition.
(commander of a naval fleet)
'largely respect a trainee' is the wordplay.
'largely' means to remove the last letter (most but not all of the letters).
'respect' becomes 'admire' (admiring is a kind of respecting).
'trainee' becomes 'l' (as in an L-plate for a learner driver).
'admire' with its last letter taken away is 'admir'.
'admir'+'a'+'l'='ADMIRAL'
(Other definitions for admiral that I've seen before include "High-ranking naval officer like Nelson" , "Navy chief" , "Senior officer; mad liar (anag.)" , "Senior naval officer" , "[NELSON] perhaps" .)