Vessels seen by navy entering bay for celebration north of the border (5,5)
I believe the answer is:
burns night
'celebration north of the border' is the definition.
(Scottish festival)
'vessels seen by navy entering bay' is the wordplay.
'vessels' becomes 'urns' (urn is a kind of vessel**).
'seen by' says to put letters next to each other.
'navy' becomes 'n' (common abbreviation eg in RN Royal Navy).
'entering' means one lot of letters goes inside another.
'bay' becomes 'bight' (bight is a kind of bay).
'urns'+'n'='urnsn'
'urnsn' placed into 'bight' is 'BURNS NIGHT'.
'for' is the link.
(Other definitions for burns night that I've seen before include "winter evening" , "January celebration in Scotland" , "January 25 for Scots" , "tribute to poet" .)