Thief may be good person protected by sovereign (7)
I believe the answer is:
rustler
'thief' is the definition.
(rustler is a kind of thief)
'good person protected by sovereign' is the wordplay.
'good person' becomes 'st' (saint).
'protected by' means one lot of letters goes inside another.
'sovereign' becomes 'ruler' (I've seen this before).
'st' put inside 'ruler' is 'RUSTLER'.
'may be' is the link.
(Other definitions for rustler that I've seen before include "person taking stock" , "One who takes livestock" , "Cattle-thief" , "Cattle thief" , "Livestock thief" .)