Call on president, assuming envoy's content (6)
I believe the answer is:
invoke
'call on' is the definition.
(I've seen this in another clue)
'president assuming envoy's content' is the wordplay.
'president' becomes 'ike' (nickname of Dwight Eisenhower).
'assuming' means one lot of letters goes inside another (inserted letters are taken in or assumed).
'content' indicates the central letters.
The middle letters of 'envoy' are 'nvo'.
'ike' enclosing 'nvo' is 'INVOKE'.
(Other definitions for invoke that I've seen before include "Call upon in prayer, perhaps for protection" , "Call in aid" , "Call on a god or spirit" , "Call upon in prayer, as for assistance" , "Call on (a deity)" .)