Sadly I am taken in by the false identity (5)
I believe the answer is:
alias
'the false identity' is the definition.
(I've seen this before)
'sadly i am taken in' is the wordplay.
I cannot quite see how this works, but
'i' could be 'a' and 'a' is present in the answer.
The remaining letters 'lias' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'by' is the link.
Can you help me to learn more?
(Other definitions for alias that I've seen before include "it may be assumed" , "Otherwise - an assumed name" , "one's assumed" , "Otherwise known as" , "Otherwise called" .)