Mashes pie weirdly as a result of stress (9)
I believe the answer is:
emphasise
'stress' is the definition.
(I know that stress can be written as emphasise)
'mashes pie weirdly' is the wordplay.
'weirdly' indicates anagramming the letters.
'mashes'+'pie'='mashespie'
'mashespie' is an anagram of 'EMPHASISE'.
'as a result of' is the link.
(Other definitions for emphasise that I've seen before include "Give extra weight to, stress" , "Stress, underline" , "Place stress on" , "Mashes pie (anag.)" , "Give extra importance to" .)