Crawl underneath large shelf (5)
I believe the answer is:
ledge
'shelf' is the definition.
(I know that shelf can be written as ledge)
'crawl underneath large' is the wordplay.
'crawl' becomes 'edge' (edge can mean to crawl along).
'underneath' is a charade indicator (letters next to each other) (in a down clue, some letters go under others).
'large' becomes 'L' (eg in clothes sizes).
'edge' after 'l' is 'LEDGE'.
(Other definitions for ledge that I've seen before include "Projecting shelf, horizontal projection" , "Narrow shelf, often attached to window" , "Submerged underwater shelf" , "Ridge or shelf" , "found halfway up a cliff?" .)