Turn white when the French skewer German composer (6)
I believe the answer is:
bleach
'turn white' is the definition.
(bleaching is a kind of turning white)
'the french skewer german composer' is the wordplay.
'the french' becomes 'le' ('the' in French).
'skewer' indicates putting letters inside (inserted letters skewer into others).
'german composer' becomes 'bach' (I've seen this before).
'le' put within 'bach' is 'BLEACH'.
'when' acts as a link.
(Other definitions for bleach that I've seen before include "Disinfectant and whitener" , "Household cleaner" , "Sterilising whitener" , "Whiten by sunlight or chemicals" , "Make colourless" .)