Small vessel makes crossing (4)
I believe the answer is:
span
'crossing' is the definition.
'small vessel' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'vessel' becomes 'pan' (I have seen 'Shallow vessel ' mean 'pan' so perhaps 'vessel' could also mean 'pan'**).
's'+'pan'='SPAN'
'makes' acts as a link.
(Other definitions for span that I've seen before include "Complete duration" , "Bridge - rotated" , "Go across" , "Extend from end to end, say bridge" , "Distance or interval between two points" .)