Extremely responsible chap, one standing in for monarch (6)
I believe the answer is:
regent
'one standing in for monarch' is the definition.
(a regent rules on behalf of a monarch)
'extremely responsible chap' is the wordplay.
'extremely' suggests removing the centre.
'chap' becomes 'gent' (I've seen this in other clues**).
'responsible' with its middle taken out is 're'.
're'+'gent'='REGENT'
(Other definitions for regent that I've seen before include "Ruler appointed when monarch is too young or away" , "acting chief" , "Not quite king" , "Ruler standing in for monarch" , "Interim monarch" .)