Femme fatale keeps heads of idiotic rich earls in tin (5)
I believe the answer is:
siren
'femme fatale' is the definition.
(I've seen this before)
'heads of idiotic rich earls in tin' is the wordplay.
'heads of' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'tin' becomes 'sn' (Sn is the chemical symbol for tin).
The first letters of 'idiotic rich earls' is 'ire'.
'ire' put inside 'sn' is 'SIREN'.
'keeps' is the link.
(Other definitions for siren that I've seen before include "Bewitching singer,part- woman, part-bird" , "She tempts" , "Temptress; warning signal" , "Risen like Lorelei" , "Device making a loud wailing sound" .)