Crossing over a period of days (4)
I believe the answer is:
ford
'crossing' is the definition.
(synonyms)
'over a period of days' is the wordplay.
'over' means one lot of letters go next to another.
'a period of' becomes 'for' (I am not sure about this - if you are sure you should believe this answer much more).
'days' becomes 'd' (abbreviation for days).
'for' put next to 'd' is 'FORD'.
Can you help me to learn more?
(Other definitions for ford that I've seen before include "Shallow part of river for crossing" , "Shallow river-crossing" , "Shallow area in a stream where it can be crossed" , "White paint for the face" , "River or stream crossing" .)