A large number gets isolated (5)
I believe the answer is:
alone
'isolated' is the definition.
(similar in meaning)
'a large number' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'number' becomes 'one' (one is a kind of number).
'a'+'l'+'one'='ALONE'
'gets' acts as a link.
(Other definitions for alone that I've seen before include "Without friends" , "without another" , "Isolated from others" , "Lacking company, solitary" , "Single" .)