It is sworn in so a thought should be spared (4)
I believe the answer is:
oath
'it is sworn' is the definition.
The definition suggests a singular noun which matches the answer.
'so a thought should be spared' is the wordplay.
'so' becomes 'oa' (I can't explain this - if you can you should believe this answer much more).
'should' becomes 'ought' (synonyms).
'be spared' is a deletion indicator.
'thought' with 'ought' removed is 'th'.
'oa'+'th'='OATH'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for oath that I've seen before include "Vow; profanity" , "One sworn" , "What frustrated people say" , "Vow or expletive" , "swearing" .)