Crossing in secret on it at sea (12)
I believe the answer is:
intersection
'crossing' is the definition.
(I know that crossing can be written as intersection)
'in secret on it at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'secret'+'on'+'it'='secretonit'
'secretonit' anagrammed gives 'tersection'.
'in'+'tersection'='INTERSECTION'
(Other definitions for intersection that I've seen before include "Crossing - junction" , "Motorway junction" , "Crossing point" , "Where two lines cross" .)