Notice small container (4)
I believe the answer is:
spot
'notice' is the definition.
(spot can mean to notice or observe)
'small container' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'container' becomes 'pot' (pot is a kind of container).
's'+'pot'='SPOT'
(Other definitions for spot that I've seen before include "Small mark or stain" , "See; place" , "Dot, stain" , "[NIGHTCLUB]" , "Observe - locality" .)