Storage space that’s left frequently (4)
I believe the answer is:
loft
'storage space' is the definition.
'left frequently' is the wordplay.
'left' becomes 'l' (common abbreviation).
'frequently' becomes 'oft' (I've seen this before).
'l'+'oft'='LOFT'
'that's' acts as a link.
(Other definitions for loft that I've seen before include "Storage space under a roof" , "Roof space area" , "Propel through the air" , "Kick high in the air" , "Space under barn roof for storing hay" .)