Win back (6)
(Other definitions for regain that I've seen before include "Recover, retrieve" , "Get back, recover the use of" , "Take back (the Ashes, for example)" , "Win back" , "Possess once more for regina" .)
'win back' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for recoup that I've seen before include "Make good a loss" , "Regain (something lost)" , "Get back one's losses" , "Recover - retrieve" , "Regain (expenditure)" .)