Charles built support after working with mentor initially (7)
I believe the answer is:
monarch
'charles' is the definition.
(King Charles)
'built support after working with mentor initially' is the wordplay.
'built support' becomes 'arch' (support in a building).
'after' says to put letters next to each other.
'working' becomes 'on' (on can mean working or activated).
'with' is a charade indicator (letters next to each other).
'initially' indicates taking the first letters.
The initial letter of 'mentor' is 'm'.
'on' put after 'm' is 'mon'.
'arch' put after 'mon' is 'MONARCH'.
(Other definitions for monarch that I've seen before include "Sole hereditary ruler" , "one over realm?" , "Sovereign; butterfly" , "Stag in Landseer picture" , "Crowned head" .)