Leo, NBC perhaps, is in a mass (2,4)
I believe the answer is:
en bloc
'in a mass' is the definition.
I can't judge whether this definition defines the answer.
'leo nbc perhaps' is the wordplay.
'perhaps' is an anagram indicator.
'leo'+'nbc'='leonbc'
'leonbc' is an anagram of 'EN BLOC'.
'is' is the link.
Can you help me to learn more?
(Other definitions for en bloc that I've seen before include "All together - as a whole" , "As a whole (body)" , "As a group (French)" , "As a group (Fr.)" , "in wholesale fashion" .)