A department finishing the festivity with stomach trouble (7)
I believe the answer is:
acidity
'stomach trouble' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe there's an association between them I don't understand?
'a department finishing the festivity' is the wordplay.
I cannot quite see how this works, but
'a' is found in the answer.
'department' could be 'd' (abbreviation eg in MOD Ministry of Defence) and 'd' is found in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'with' is the link.
Can you help me to learn more?
(Other definitions for acidity that I've seen before include "Bitterness" , "Tartness" , "bite" , "Sourness, sharpness" , "Tart quality" .)