Cleaner vehicle filled with hydrogen (4)
I believe the answer is:
char
'cleaner' is the definition.
(charwoman is a type of cleaner)
'vehicle filled with hydrogen' is the wordplay.
'vehicle' becomes 'car' (car is a kind of vehicle).
'filled with' is an insertion indicator.
'hydrogen' becomes 'H' (H is the chemical symbol for hydrogen).
'car' placed around 'h' is 'CHAR'.
(Other definitions for char that I've seen before include "House cleaner" , "One cleans" , "Burn; cleaner; fish" , "Clean < fish" , "Burn; scorch" .)