Strange condition — one remains if twin daughters and sons leave (7)
I believe the answer is:
oddness
'one remains if twin daughters and sons leave' is the definition.
I can't tell whether this definition defines the answer.
'strange condition' is the wordplay.
'strange' becomes 'odd' (I've seen this before).
'condition' becomes 'ness' (I am not sure about this - if you are sure you should believe this answer much more).
'odd'+'ness'='ODDNESS'
Can you help me to learn more?
(Other definitions for oddness that I've seen before include "Eccentricity" , "Peculiar quality" , "Peculiarity characteristic" , "Curious state" , "It isn't even" .)