Historic garment design getting no initial surprised reaction (6,4)
I believe the answer is:
double take
'surprised reaction' is the definition.
(a double take indicates surprise)
'historic garment design getting no initial' is the wordplay.
'historic garment' becomes 'doublet' (historical type of jacket).
'design' becomes 'make'.
'getting no initial' means to remove the first letter.
'make' with its initial letter taken away is 'ake'.
'doublet'+'ake'='DOUBLE TAKE'
(Other definitions for double take that I've seen before include "Delayed action" , "Second reaction to the unexpected" .)