Occupy Vietnam? US endeavoured, to an extent (5)
I believe the answer is:
amuse
'occupy' is the definition.
(thesaurus)
'vietnam? us endeavoured to an extent' is the wordplay.
'to an extent' says the answer is hidden in the clue.
'AMUSE' is hidden in the letters of 'vietnam us endeavoured'.
(Other definitions for amuse that I've seen before include "Cause mirth - distract" , "interest" , "Delight" , "Pleasantly occupy" , "Divert or entertain mildly" .)