Saint, fifty, in love — well! (8)
I believe the answer is:
honestly
'well' is the definition.
(to live well is to live honestly or ethically)
'saint fifty in love' is the wordplay.
'saint' becomes 'st'.
'fifty' becomes 'L' (Roman numeral).
'in' indicates putting letters inside.
'love' becomes 'honey' (synonyms).
'st'+'l'='stl'
'stl' inserted inside 'honey' is 'HONESTLY'.
(Other definitions for honestly that I've seen before include "No kidding!" , "In a truthful way" , "Fairly" , "On the sly (anag.)" , "How law-abiding people act" .)