Pineapple cocktail mixer's not cool (7)
I believe the answer is:
grenade
'pineapple' is the definition.
(I've seen this before)
'cocktail mixer's not cool' is the wordplay.
'cocktail mixer' becomes 'grenadine'.
'not' suggests deleting specific letters.
'cool' becomes 'in' (I've seen this in other clues).
'grenadine' with 'in' taken away is 'GRENADE'.
(Other definitions for grenade that I've seen before include "Small hand missile" , "Small explosive device" , "Small bomb thrown by hand" , "ultimate weapon" , "Enraged (anag)" .)