Man joins affair to be safe (4,2,4)
I believe the answer is:
just in case
'to be safe' is the definition.
(I've seen this in another clue)
'man joins affair' is the wordplay.
'man' becomes 'justin' ().
'joins' is a charade indicator (letters next to each other).
'affair' becomes 'case'.
'justin'+'case'='JUST IN CASE'
(Other definitions for just in case that I've seen before include "how one should be when judging suit?" , "In the unlikely event of problems" , "as a precaution" , "the judge needs to be" , "Lest" .)