Adult captured by RN fled (3)
I believe the answer is:
ran
'fled' is the definition.
(ran away)
'adult captured by rn' is the wordplay.
'adult' becomes 'a' (abbreviation).
'captured by' indicates putting letters inside.
'a' placed into 'rn' is 'RAN'.
(Other definitions for ran that I've seen before include "Moved fast or managed" , "Directed - fled" , "was in charge" , "travelled fast" , "Took a single" .)