Counterpart (4)
There are a few possibilities for this:
parallel
'counterpart' is the definition.
(I know that counterpart can be written as twin)
This is all the clue.
(Other definitions for twin that I've seen before include "indistinguishable from the next?" , "Identical sibling" , "Dead ringer" , "Make association" , "Very much like" .)
(Other definitions for oppo that I've seen before include "China" , "Mate" , "Counterpart; rival" , "Opposite number (colloq.)" , "Colleague, friend (colloq.)" .)
'counterpart' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for parallel that I've seen before include "equivalent" , "'Alongside something, it's all pearl (8)'" , "Imaginary line around the Earth" , "suggesting inability to make projected meeting" , "Imaginary line round the Earth" .)