What's concerning man who substitutes for ruler? (6)
I believe the answer is:
regent
'who substitutes for ruler?' is the definition.
(a regent rules on behalf of another)
'what's concerning man' is the wordplay.
'what's concerning' becomes 're' (both can mean on a particular topic).
'man' becomes 'gent' (gent is a kind of man).
're'+'gent'='REGENT'
(Other definitions for regent that I've seen before include "One acting on behalf of an incapacitated monarch" , "Ruler standing in for monarch" , "Closely; almost" , "acting chief" , "Person who rules for a sovereign" .)