Starr initially is not what Lennon, McCartney, or Harrison nominally is (5)
I believe the answer is:
saint
'what lennon mccartney or harrison nominally is' is the definition.
I can't tell whether this defines the answer.
'starr initially is not' is the wordplay.
'initially' indicates taking the first letters.
'is not' becomes 'ain't' (non-standard version of 'isn't').
The initial letter of 'starr' is 's'.
's'+'aint'='SAINT'
Can you help me to learn more?
(Other definitions for saint that I've seen before include "Holy man" , "See 6" , "Very virtuous person" , "paragon!" , "Beatified person" .)