Fifty on the verge of the shelf (5)
I believe the answer is:
ledge
'the shelf' is the definition.
('ledge' can be a synonym of 'shelf')
'fifty on the verge' is the wordplay.
'fifty' becomes 'L' (Roman numeral).
'on' means one lot of letters go next to another.
'the verge' becomes 'edge' (I've seen this in another clue).
'l'+'edge'='LEDGE'
'of' is the link.
(Other definitions for ledge that I've seen before include "Narrow projecting shelf as from window" , "Rock shelf" , "Narrow projecting shelf, on cliff or building" , "Shelf-like projection" , "found halfway up a cliff?" .)