Fools with missing friends (6)
I believe the answer is:
allies
'friends' is the definition.
('ally' can be a synonym of 'friend')
'fools with missing' is the wordplay.
'fools' becomes 'wallies' (wally can mean a stupid person).
'with' becomes 'w' (abbreviation).
'missing' indicates named letters should be taken away.
'wallies' with 'w' removed is 'ALLIES'.
(Other definitions for allies that I've seen before include "Joins forces" , "Friends in a cause" , "People or nations joined against an enemy" , "They are banded together for mutual support" , "Countries that opposed Hitler" .)