Therefore, having an identical twin is not bad (2-2)
I believe the answer is:
so-so
'not bad' is the definition.
(I've seen this before)
'therefore having an identical twin' is the wordplay.
I cannot quite understand how this works, but
'therefore' could be 'so' ('so' can mean 'consequently' or 'therefore') and 'so' is found within the answer.
The remaining letters 'so' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'is' is the link.
Can you help me to learn more?
(Other definitions for so-so that I've seen before include "Indifferently good" , "Nothing to write home about" , "Barely passable" , "OK, but nothing special" , "Of middling quality" .)