Yankee coming to attention in revolutionary period (4)
I believe the answer is:
year
'revolutionary period' is the definition.
(the Earth completing a revolution around the Sun)
'yankee coming to attention' is the wordplay.
'yankee' becomes 'y' (phonetic alphabet: alpha, bravo, charlie etc.).
'coming to' means one lot of letters go next to another.
'attention' becomes 'ear' (ear is a kind of attention).
'y'+'ear'='YEAR'
'in' is the link.
(Other definitions for year that I've seen before include "Pupil group" , "Group of students" , "Two semesters" , "Period of just over 52 weeks" , "1066, for example" .)