Label smaller than a postage stamp! (3)
I believe the answer is:
tag
'label' is the definition.
(tag is a kind of label)
'smaller than a postage stamp' is the wordplay.
'TAG' is hidden within the letters.
However, I'm unsure how the hidden word is indicated.
Can you help me to learn more?
(Other definitions for tag that I've seen before include "Label or sticker" , "stock quotation" , "Ticket" , "dog" , "Identity disc" .)