Notice small vessel (4)
I believe the answer is:
spot
'notice' is the definition.
(thesaurus)
'small vessel' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vessel' becomes 'pot' (pot is a kind of vessel**).
's'+'pot'='SPOT'
(Other definitions for spot that I've seen before include "Informally, catch sight of" , "Pimple; place" , "place on TV programme" , "Dot, stain" , "Small mark or stain" .)