Fern's underwear caught by Barbie's partner (7)
I believe the answer is:
bracken
'fern's' is the definition.
(type of fern)
'underwear caught by barbie's partner' is the wordplay.
'underwear' becomes 'bra' (bra is a type of undergarment).
'caught' becomes 'c' (cricket abbreviation).
'by' is a charade indicator (letters next to each other).
'barbie's partner' becomes 'Ken' (boyfriend of the Barbie doll).
'bra'+'c'+'ken'='BRACKEN'
(Other definitions for bracken that I've seen before include "A mass of wild ferns" , "Plant" , "Mass of ferns on hillside" , "Fern with large fronds" , "Type of fern" .)