Extend across top of seagoing vessel (4)
I believe the answer is:
span
'extend across' is the definition.
(I've seen this in another clue)
'top of seagoing vessel' is the wordplay.
'top of' suggests taking the first letters.
'vessel' becomes 'pan' (vessel or container for cooking).
The first letter of 'seagoing' is 's'.
's'+'pan'='SPAN'
(Other definitions for span that I've seen before include "Go across" , "Stretch; traverse" , "Bridge or range" , "Cover" , "Interval; pair of yoked oxen" .)