Reckoned diets managed to involve friend (9)
I believe the answer is:
estimated
'reckoned' is the definition.
('estimate' can be a synonym of 'reckon')
'diets managed to involve friend' is the wordplay.
'managed' indicates anagramming the letters.
'to involve' indicates putting letters inside (inserted letters get involved).
'friend' becomes 'mate' (mate is a kind of friend).
'diets' with letters rearranged gives 'estid'.
'estid' enclosing 'mate' is 'ESTIMATED'.
(Other definitions for estimated that I've seen before include "Calculated roughly" , "Judged roughly" , "Calculated approximately" , "Guessed" , "Rough" .)