Sandwich, though extremely tiny (5)
I believe the answer is:
butty
'sandwich' is the definition.
(I've seen this before)
'though extremely tiny' is the wordplay.
'though' becomes 'but'.
'extremely' suggests removing the centre.
'tiny' with its centre taken out is 'ty'.
'but'+'ty'='BUTTY'
(Other definitions for butty that I've seen before include "something to eat" , "Partner" , "Sandwich; freight barge" , "Sandwich; towed barge" .)