Fashion model (4)
This clue is a double definition.
'fashion' is the first definition.
(both can mean to shape something for a purpose)
'model' is the second definition.
(both can mean to mould or shape)
(Other definitions for form that I've seen before include "Configuration; type" , "Visible shape, class" , "Seat" , "etiquette" , "Printed document with spaces to write" .)
'fashion model' is the definition.
(synonyms)
This is all the clue.
(Other definitions for mannequin that I've seen before include "Dummy figure" , "Display dummy" , "Shop's dummy" , "Life-size dummy for displaying clothes" , "fashion model" .)