Narrow issue that's a collector's item (7,7)
I believe the answer is:
limited edition
'a collector's item' is the definition.
(eg a limited edition book might be a collector's i)
'narrow issue' is the wordplay.
'narrow' becomes 'limited' (associated in meaning).
'issue' becomes 'edition' (edition is a kind of issue).
'limited'+'edition'='LIMITED EDITION'
'that's' is the link.
(Other definitions for limited edition that I've seen before include "Rare book" , "Small number of copies" , "An issue of a fixed number" .)