Large number left secure (4)
I believe the answer is:
seal
'secure' is the definition.
(I've seen this in another clue)
'large number left' is the wordplay.
'large number' becomes 'sea' (I've seen this in another clue).
'left' becomes 'l' (common abbreviation).
'sea'+'l'='SEAL'
(Other definitions for seal that I've seen before include "Stamped wax; mammal" , "Stamp of guarantee" , "Stick down" , "Bull or cow" , "Secure; aquatic animal" .)