Date boxes in European river (5)
I believe the answer is:
seine
'european river' is the definition.
(I've seen this in another clue)
'date boxes in' is the wordplay.
'date' becomes 'see' (synonyms).
'boxes' is an insertion indicator (the outside letters must 'box' inside letters in).
'see' going around 'in' is 'SEINE'.
(Other definitions for seine that I've seen before include "Parisian river" , "Second longest river in France" , "Paris river" , "French river -- type of fishing net" , "Paris's river" .)