Yankee parking Sierra -- European knocked over notice (4)
I believe the answer is:
'notice' is the definition.
(I've seen this before)
'yankee parking sierra european knocked over' is the wordplay.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'parking' becomes 'p' (abbreviation on road signs).
'sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'european' becomes 'E' (abbreviation e.g. EU).
'knocked over' says the letters should be written in reverse.
'ypse' reversed gives 'ESPY'.
(Other definitions for espy that I've seen before include "Catch a glimpse of" , "Spot, notice" , "Observe, notice" , "Perceive" , "Spot; observe" .)