Baby's bed needing heavyweight fabric (6)
I believe the answer is:
cotton
'fabric' is the definition.
(I know that cotton is a type of fabric)
'baby's bed needing heavyweight' is the wordplay.
'baby's bed' becomes 'cot' (cot is a kind of baby's bed).
'needing' says to put letters next to each other.
'heavyweight' becomes 'ton' (a ton weighs a lot).
'cot'+'ton'='COTTON'
(Other definitions for cotton that I've seen before include "Source of cloth" , "Golfing man" , "Plant fibre for fabrics" , "Old golfer" , "Fabric (of natural fibre)" .)