A department finishing the festivity with stomach trouble (7)
I believe the answer is:
acidity
'stomach trouble' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'a department finishing the festivity' is the wordplay.
I cannot really understand how this works, but
'a' is found in the answer.
'department' could be 'd' (abbreviation eg in MOD Ministry of Defence) and 'd' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
This explanation may well be incorrect...
'with' acts as a 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" .)