Hardly an extensive record collection! (6,4)
I believe the answer is:
single file
'hardly an' is the definition.
I can't judge whether this definition defines the answer.
'extensive record collection' is the wordplay.
'extensive' becomes 'single' (I am not sure about this - if you are sure you should believe this answer much more).
'record collection' becomes 'file' (I've seen this in another clue).
'single'+'file'='SINGLE-FILE'
Can you help me to learn more?
(Other definitions for single file that I've seen before include "Only the one document" , "Contents of minimal archive" , "unique dossier" , "Progress so narrowly" , "Where to keep unmarried person's records" .)