Safe signal fully understood (3,5)
I believe the answer is:
all clear
'safe signal' is the definition.
(the all clear signal is given when it's safe)
'fully understood' is the wordplay.
'fully' becomes 'all' (I've seen this in another clue).
'understood' becomes 'clear' (is that clear?).
'all'+'clear'='ALL-CLEAR'
(Other definitions for all clear that I've seen before include "no longer misunderstood?" , "Signal that danger is past" , "ARP signal" , "no doubts" , "Signal to resume normal activities" .)