Root taking part in mixed doubles (4)
I believe the answer is:
eddo
'root' is the definition.
(I know that eddo is a type of root)
'part in mixed doubles' is the wordplay.
'part in' indicates the answer is hidden within the clue.
'EDDO' can be found hidden inside 'mixed doubles'.
'taking' acts as a link.
(Other definitions for eddo that I've seen before include "Edible corm" , "Edible plant" , "Tropical plant" , "taro tuber" , "elephant's ear" .)