In secret I'd developed rash (10)
I believe the answer is:
indiscreet
'rash' is the definition.
(thesaurus)
'in secret i'd developed' is the wordplay.
'developed' indicates anagramming the letters.
'secret'+'id'='secretid'
'secretid' anagrammed gives 'discreet'.
'in'+'discreet'='INDISCREET'
(Other definitions for indiscreet that I've seen before include "Tactless" , "Imprudent -- 10 across (anag)" , "Too ready to reveal secret things" , "Too ready to tell secrets" , "shameless" .)