Factories making line in underwear (6)
I believe the answer is:
plants
'factories' is the definition.
(manufacturing plants)
'line in underwear' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'in' is an insertion indicator.
'underwear' becomes 'pants' (underpants).
'l' placed within 'pants' is 'PLANTS'.
'making' is the link.
(Other definitions for plants that I've seen before include "Living organisms without power of movement" , "Sets seeds in the ground" , "climbers?" , "Places in the earth" , "Settles" .)