Petty Officer left out initially. Some neck! (4)
I believe the answer is:
polo
'some neck' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'petty officer left out initially' is the wordplay.
'petty officer' becomes 'po' (naval abbreviation).
'initially' suggests taking the first letters.
The initial letters of 'left out' is 'lo'.
'po'+'lo'='POLO'
Can you help me to learn more?
(Other definitions for polo that I've seen before include "car?" , "Horseback sport" , "Pool (anag.)" , "Equestrian game" , "Top" .)