So this is pickled and not allowed to go to waste (4)
I believe the answer is:
used
'so this is pickled' is the definition.
The definition suggests a singular noun but the answer is not.
'not allowed to go to waste' is the wordplay.
'not allowed' becomes 'out' (I've seen this before).
'go' is a deletion indicator.
'to waste' becomes 'sed' (I can't explain this - if you can you should give a lot more credence to this answer).
'out' with 'to' taken out is 'u'.
'u'+'sed'='USED'
'and' is the link.
Can you help me to learn more?
(Other definitions for used that I've seen before include "Exploited group" , "worked unfairly" , "Brought into play" , "Old" , "exhausted" .)