Beam's losing head seeing dessert (6)
I believe the answer is:
afters
'dessert' is the definition.
(afters can mean a dessert course)
'beam's losing head' is the wordplay.
'beam's' becomes 'rafter's' (rafter is a kind of beam).
'losing head' means to remove the first letter.
'rafters' with its first letter removed is 'AFTERS'.
'seeing' acts as a link.
(Other definitions for afters that I've seen before include "Tiramisu, possibly" , "Dessert course" , "Pud" , "food for pudding" , "Informal desserts, puddings" .)