Man, say, is extremely large (4)
I believe the answer is:
isle
'man say' is the definition.
(Isle of Man is an example)
'is extremely large' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'large' with its middle taken out is 'le'.
'is'+'le'='ISLE'
(Other definitions for isle that I've seen before include "See 10 Down" , "Isolated land" , "eg Hoy, Yell or Sark" , "Small bit of land surrounded by water" , "Ait" .)