Bridge of small vessel (4)
I believe the answer is:
span
'bridge' is the definition.
(I know that bridge can be written as span)
'small vessel' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vessel' becomes 'pan' (vessel or container for cooking).
's'+'pan'='SPAN'
'of' is the link.
(Other definitions for span that I've seen before include "Length of bridge - 9 inches!" , "Duration, length" , "Team of two matched oxen" , "Extent - 9 inches" , "Width of wings" .)