Decide on right straight-edge (5)
I believe the answer is:
ruler
'edge' is the definition.
(I have seen 'Straight edge ' mean 'ruler' so perhaps 'edge' could also mean 'ruler')
'decide on right' is the wordplay.
'decide' becomes 'rule' (ruling is a kind of deciding).
'on' says to put letters next to each other.
'right' becomes 'r' (common abbreviation).
'rule'+'r'='RULER'
'straight' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for ruler that I've seen before include "Potentate" , "Monarch - drafting tool" , "Person in charge of country" , "establishing a line?" , "Chief" .)