50 reaching the limit of the shelf (5)
I believe the answer is:
ledge
'the shelf' is the definition.
('ledge' can be a synonym of 'shelf')
'50 reaching the limit' is the wordplay.
'50' becomes 'l' (Roman numeral).
'reaching' says to put letters next to each other.
'the limit' 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 "Shelf-life projection" , "Narrow shelf, often attached to window" , "Projecting ridge on a mountain" , "Submerged underwater shelf" , "Ability to contain one's emotions" .)