Combination lock in case it is left undone (7)
I believe the answer is:
backlog
'it is left undone' is the definition.
The definition suggests a singular noun which matches the answer.
'combination lock in case' is the wordplay.
'combination' indicates an anagram (I've seen 'combination of' mean this (I've seen this in other clues)).
'in' means one lot of letters goes inside another.
'case' becomes 'bag' (bag is a kind of case).
'lock' is an anagram of 'cklo'.
'cklo' going within 'bag' is 'BACKLOG'.
Can you help me to learn more?
(Other definitions for backlog that I've seen before include "Accumulation of work waiting to be done" , "Work awaiting attention" , "Amount of uncompleted work" , "Quantity of unfinished work" , "work not yet completed" .)