Chamber to expand endlessly when storing too much! (6)
I believe the answer is:
grotto
'chamber' is the definition.
(type of chamber or cave)
'expand endlessly when storing too much' is the wordplay.
'expand' becomes 'grow' (to grow is to expand in size).
'endlessly' means to remove the last letter.
'when storing' means one lot of letters goes inside another.
'too much' becomes 'OTT' (short for over the top).
'grow' with its last letter removed is 'gro'.
'gro' going around 'ott' is 'GROTTO'.
'to' is the link.
(Other definitions for grotto that I've seen before include "(Imitation) cave" , "Attractive natural, or artificial, cave" , "Fancy cave" , "Santa's cave" , "Pretty cave" .)