Temporary head of state is concerning chap (6)
I believe the answer is:
regent
'temporary head of state' is the definition.
(a regent rules on behalf of a monarch)
'concerning chap' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'chap' becomes 'gent' (I've seen this in other clues).
're'+'gent'='REGENT'
'is' is the link.
(Other definitions for regent that I've seen before include "Ruler in the absence of a monarch" , "Ruler during monarch's minority" , "Someone ruling on behalf of another" , "Acting ruler" , "Ruler standing in for monarch" .)