Everyone easily understood signal that danger is over (3-5)
I believe the answer is:
all clear
'signal that danger is over' is the definition.
(I've seen this before)
'everyone easily understood' is the wordplay.
'everyone' becomes 'all' (all people).
'easily understood' becomes 'clear' (I've seen this before).
'all'+'clear'='ALL-CLEAR'
(Other definitions for all clear that I've seen before include "no longer misunderstood?" , "Siren signal that bombing has ended" , "Signal that it's safe to come out" , "without any ambiguity" , "End-of-danger signal" .)