Not all respect a large naval officer (7)
I believe the answer is:
admiral
'naval officer' is the definition.
(commander of a naval fleet)
'not all respect a large' is the wordplay.
'not all' means to remove the last letter (not quite all of it).
'respect' becomes 'admire' (admiring is a kind of respecting).
'a large' becomes 'al' (I can't justify this - if you can you should give a lot more credence to this answer).
'admire' with its final letter removed is 'admir'.
'admir'+'al'='ADMIRAL'
Can you help me to learn more?
(Other definitions for admiral that I've seen before include "Highest ranking naval officer" , "Blake, for one" , "Senior officer; mad liar (anag.)" , "Naval high-up" , "Naval chief" .)