Very small container with unknown quantity (4)
I believe the answer is:
tiny
'very small' is the definition.
(I've seen this before)
'container with unknown quantity' is the wordplay.
'container' becomes 'tin' (tin is a kind of container**).
'with' means one lot of letters go next to another.
'unknown quantity' becomes 'y' (algebra).
'tin'+'y'='TINY'
(Other definitions for tiny that I've seen before include "- - Tim (Dickens)" , "Really small" , "with sharp points?" , "Minute, diminutive" , "pocket" .)