One takes strange faithless lass out (5)
I believe the answer is:
thief
'one' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'strange faithless lass out' is the wordplay.
'strange' is an anagram indicator.
'out' is a deletion indicator.
'faithless' with 'lass' taken out is 'fithe'.
'fithe' anagrammed gives 'THIEF'.
'takes' is the link.
Can you help me to learn more?
(Other definitions for thief that I've seen before include "nicker?" , "He takes" , "Afternoon break" , "One who is fourth in line after a rich man" , "Larcenist" .)