One captured by RN fled (3)
I believe the answer is:
ran
'fled' is the definition.
(ran away)
'one captured by rn' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'captured by' is an insertion indicator.
'a' going into 'rn' is 'RAN'.
(Other definitions for ran that I've seen before include "Administered" , "Moved fast or managed" , "Took to one's heels" , "Entered an election or managed" , "Sped" .)