Confident, like retiring American left-winger (7)
I believe the answer is:
assured
'confident' is the definition.
('assured' can be similar in meaning to 'confident')
'like retiring american left-winger' is the wordplay.
'like' becomes 'as'.
'retiring' says the letters should be written in reverse.
'american' becomes 'us'.
'left-winger' becomes 'red' (red is a kind of left-winger).
'us' in reverse letter order is 'su'.
'as'+'su'+'red'='ASSURED'
(Other definitions for assured that I've seen before include "Confident - certain" , "Gave confidence to" , "Confident, secure" , "Informed with certainty and confidence" , "Confident and self-possessed" .)