Sailor not at home? That’s concerning (5)
I believe the answer is:
about
'concerning' is the definition.
(about can mean concerning or relating to)
'sailor not at home?' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'not at home?' becomes 'out' (out can mean away from home).
'ab'+'out'='ABOUT'
'that's' is the link.
(Other definitions for about that I've seen before include "in the offing?" , "nearby" , "Regarding" , "In the area of or in relation to" , "On the topic of" .)