English king back on throne's beginning to make an effort (5)
I believe the answer is:
exert
'make an effort' is the definition.
(exerting oneself by making a special effort)
'english king back on throne's beginning' is the wordplay.
'english' becomes 'E' (abbreviation).
'king' becomes 'rex' (I've seen this before).
'back' is a reversal indicator.
'on' says to put letters next to each other.
'beginning' suggests taking the first letters.
The first letter of 'thrones' is 't'.
'rex' in reverse letter order is 'xer'.
'e'+'xer'+'t'='EXERT'
'to' acts as a link.
(Other definitions for exert that I've seen before include "Wield - apply" , "Work strenuously" , "Employ" , "Bring into play, wield" , "Strain" .)