Henry Percy developed a secret code (6)
I believe the answer is:
cypher
'a secret code' is the definition.
('cypher' can be a synonym of 'secret code')
'henry percy developed' is the wordplay.
'henry' becomes 'h' ('H' can be a synonym of 'henry').
'developed' is an anagram indicator.
'h'+'percy'='hpercy'
'hpercy' anagrammed gives 'CYPHER'.
(Other definitions for cypher that I've seen before include "Secret code (variant spelling)" , "Code; unimportant person" , "Secret way of writing" .)