Not as large, empty vessel (4)
I believe the answer is:
less
'not as' is the definition.
(eg 'not as tall' means 'less tall')
'large empty vessel' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'empty' becomes 'e' (abbreviation e.g. on guages).
'vessel' becomes 'SS' (prefix in ship names eg SS Great Britain).
'l'+'e'+'ss'='LESS'
(Other definitions for less that I've seen before include "Smaller in amout or degree" , "A smaller amount" , "Comparatively little" , "Fewer; minus" , "Fewer in number" .)