Hood put on girl's head when dressing? (6)
I believe the answer is:
robing
'dressing?' is the definition.
(robing is a kind of dressing)
'hood put on girl's head' is the wordplay.
'hood' becomes 'Robin' (Robin Hood).
'put on' says to put letters next to each other.
'head' says to take the initial letters.
The first letter of 'girl' is 'g'.
'robin'+'g'='ROBING'
'when' acts as a link.
(Other definitions for robing that I've seen before include "dressing up" , "clothing" .)