As PM he is somehow last to acknowledge stress (9)
I believe the answer is:
emphasise
'stress' is the definition.
(I know that stress can be written as emphasise)
'as pm he is somehow last to acknowledge' is the wordplay.
'somehow' indicates an anagram.
'last to' suggests the final letters.
The final letter of 'acknowledge' is 'e'.
'as'+'pm'+'he'+'is'='aspmheis'
'aspmheis' with letters rearranged gives 'emphasis'.
'emphasis'+'e'='EMPHASISE'
(Other definitions for emphasise that I've seen before include "Give extra weight to, stress" , "Give extra importance to" , "Mashes pie (anag.)" , "'Stress, accentuate (9)'" , "Place stress on" .)