Oh dear, I got in using a false name! (5)
I believe the answer is:
alias
'a false name' is the definition.
(synonyms)
'oh dear i got in' is the wordplay.
'oh dear' becomes 'alas' (I've seen this before).
'got in' indicates putting letters inside.
'alas' enclosing 'i' is 'ALIAS'.
'using' is the link.
(Other definitions for alias that I've seen before include "Assumed identity" , "Handle that isn't genuine" , "Otherwise known as" , "stage name, perhaps" , "False or assumed name" .)