Expert claims referee's assistant essentially committed sin (7)
I believe the answer is:
avarice
'committed sin' is the definition.
'avarice' can be an answer for 'sin' (avarice is a kind of sin). I'm unsure of the 'committed' bit.
'expert claims referee's assistant essentially' is the wordplay.
'expert' becomes 'ace' (ace can informally mean very good).
'claims' means one lot of letters goes inside another.
'referee' becomes 'VAR' (video assistant referee).
'assistant' becomes 'aid' (I've seen this before).
'essentially' means to look at the middle letters.
The middle letter of 'aid' is 'i'.
'var'+'i'='vari'
'ace' enclosing 'vari' is 'AVARICE'.
Can you help me to learn more?
(Other definitions for avarice that I've seen before include "Wanting to be rich" , "A deadly sin" , "Eager desire" , "Sway, control" , "Greed, covetousness" .)