Show amusement about a tiny amount (5)
I believe the answer is:
grain
'tiny amount' is the definition.
(I've seen this before)
'show amusement about a' is the wordplay.
'show amusement' becomes 'grin' (I've seen this in another clue).
'about' is an insertion indicator.
'grin' enclosing 'a' is 'GRAIN'.
(Other definitions for grain that I've seen before include "Structure of wood" , "cereal crop" , "Direction of fibres found in wood" , "bit of corn" , "Small particle - weight" .)