Attempt to stay safe at the moment — in support at first (4,2,4)
I believe the answer is:
stay at home
'attempt to stay safe at the moment' is the definition.
(UK Covid-19 slogan)
'in support at first' is the wordplay.
'in' becomes 'athome'.
'support' becomes 'stay' (stay is a kind of support).
'at first' says to put letters next to each other.
'athome' after 'stay' is 'STAY-AT-HOME'.
(Other definitions for stay at home that I've seen before include "Don't go out" , "Unsociable" , "Unadventurous" , "Indoor" , "Not this guy" .)