Outlaw robs prince after releasing knight errant (9)
I believe the answer is:
proscribe
'outlaw' is the definition.
(I know this)
'robs prince after releasing knight errant' is the wordplay.
'after releasing' is a deletion indicator.
'knight' becomes 'N' (chess abbreviation).
'errant' indicates an anagram (errant can mean travelling or straying).
'robs'+'prince'='robsprince'
'robsprince' with 'n' taken out is 'robsprice'.
'robsprice' with letters rearranged gives 'PROSCRIBE'.
(Other definitions for proscribe that I've seen before include "Denounce" , "Ban" , "Exile" , "Forbid; condemn" , "Prohibit" .)