Worried MP? Ease his stress (9)
I believe the answer is:
emphasise
'stress' is the definition.
('stress'->'emphasize' is in my internal thesaurus and 'ize'->'ise')
'worried mp? ease his' is the wordplay.
'worried' is an anagram indicator.
'mp'+'ease'+'his'='mpeasehis'
'mpeasehis' is an anagram of 'EMPHASISE'.
(Other definitions for emphasise that I've seen before include "Stress, underline" , "Place stress on" , "Give extra weight to, stress" , "Mashes pie (anag.)" , "Give extra importance to" .)