Ruth Rendell's first English detective (7)
I believe the answer is:
remorse
'ruth' is the definition.
(thesaurus)
'rendell's first english detective' is the wordplay.
'first' indicates taking the first letters.
'english' becomes 'e' (abbreviation).
'detective' becomes 'morse' (I've seen this before).
The first letter of 'rendell' is 'r'.
'r'+'e'+'morse'='REMORSE'
(Other definitions for remorse that I've seen before include "Gnawing pain of conscience" , "Regret or sorrow for wrong-doing" , "Pangs of conscience" , "Regret for wrong" , "A feeling of deep regret" .)