It's minuscule if not out in twenty ends (4)
I believe the answer is:
tiny
'it's minuscule' is the definition.
(I've seen this in another clue)
'not out in twenty ends' is the wordplay.
'not out' becomes 'in' ('in' is the opposite to 'out').
'in' means one lot of letters goes inside another.
'ends' suggests removing the centre.
'twenty' with its centre taken out is 'ty'.
'in' placed within 'ty' is 'TINY'.
'if' is the link.
(Other definitions for tiny that I've seen before include "- - Tim (Dickens)" , "Minuscule" , "Miniature" , "Slight" , "Really small" .)