Assistant referee’s helper books oddly trim Italian star (9)
I believe the answer is:
pavarotti
'italian star' is the definition.
I know nothing about this answer so I cannot tell whether it can be defined by this definition.
'assistant referee's helper books oddly trim' is the wordplay.
'assistant' becomes 'PA' (personal assistant).
'referee' becomes 'VAR' (video assistant referee).
'helper books' becomes 'ot' (I can't justify this - if you can you should give a lot more credence to this answer).
'oddly' indicates alternate letters (letters in odd-numbered positions).
The alternating letters of 'trim' are 'ti'.
'pa'+'var'+'ot'+'ti'='PAVAROTTI'
Can you help me to learn more?
(Other definitions for pavarotti that I've seen before include "Operatic tenor, d.2007" , "Singer" , "Luciano ......... popular modern Italian tenor." , "Air ace" , "Luciano . . . . . . . . ., great Italian tenor" .)