Party requisite causing agitation in Manhattan? (8,6)
I believe the answer is:
cocktail shaker
This clue is a double definition.
'party requisite' is the first definition.
(something used at a party)
'causing agitation in manhattan?' is the second definition.
(a cocktail shaker might shake a Manhattan cocktail)
(Other definitions for cocktail shaker that I've seen before include "Barman's mixer" , "Drinks mixer" , "kept behind bars" .)