One managing to contain phosphorus in large vessel (6)
I believe the answer is:
copper
'large vessel' is the definition.
(container formerly used for laundry)
'one managing to contain phosphorus' is the wordplay.
'one managing' becomes 'coper' (I've seen this before**).
'to contain' is an insertion indicator.
'phosphorus' becomes 'p' ('P' can be a synonym of 'phosphorus').
'coper' going around 'p' is 'COPPER'.
'in' is the link.
(Other definitions for copper that I've seen before include "English policeman" , "Metallic element and British bobby" , "Reddish-brown metallic element" , "Metal, Cu" , "Reddish-brown like colourful beech" .)