Suppose fool has turn with yours truly (6)
I believe the answer is:
assume
'suppose' is the definition.
(assume can mean to suppose or expect)
'fool has turn with yours truly' is the wordplay.
'fool' becomes 'ass' (both can mean a stupid person).
'has' is a charade indicator (letters next to each other).
'turn' becomes 'u' (a 'U-turn' is a kind of turn).
'with' says to put letters next to each other.
'yours truly' becomes 'me' (the person who set this crossword).
'ass'+'u'+'me'='ASSUME'
(Other definitions for assume that I've seen before include "Take for granted or agreed" , "Suppose; put on" , "Take (responsibility)" , "Dark brown fur" , "Take or put on" .)