I must leave master class (5)
I believe the answer is:
genus
'class' is the definition.
(I've seen this before)
'i must leave master' is the wordplay.
'must leave' suggests deleting specific letters.
'master' becomes 'genius' (genius can mean a master or expert).
'genius' with 'i' removed is 'GENUS'.
(Other definitions for genus that I've seen before include "Type" , "Latin class?" , "Category above species (Biology)" , "Biological group containing one or more species" , "group of organisms" .)