A tin containing high explosive's white (5)
I believe the answer is:
ashen
'white' is the definition.
(I know that white can be written as ashen)
'a tin containing high explosive's' is the wordplay.
'tin' becomes 'Sn' (Sn is the chemical symbol for tin).
'containing' means one lot of letters goes inside another.
'high explosive' becomes 'he'.
'sn' enclosing 'he' is 'shen'.
'a'+'shen'='ASHEN'
(Other definitions for ashen that I've seen before include "Grey-faced" , "(Of face) pale (with shock)" , "Pale (faced)" , "Pale in complexion" , "Wan - wooden - pallid" .)