A great time starting dipping into the collection! (4)
I believe the answer is:
fund
'the collection' is the definition.
(thesaurus)
'a great time starting dipping' is the wordplay.
'a great time' becomes 'fun' (I can't explain this - if you can you should believe this answer much more).
'starting' suggests taking the first letters.
The initial letter of 'dipping' is 'd'.
'fun'+'d'='FUND'
'into' acts as a link.
Can you help me to learn more?
(Other definitions for fund that I've seen before include "Provide with money, say project" , "Sponsor" , "Stock, resources" , "Reserve of money set aside for a purpose" , "Pool - float" .)