Chief in charge of what's on the agenda (5)
I believe the answer is:
topic
'the agenda' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'chief in charge of what's on' is the wordplay.
'chief' becomes 'top' ('top' can be a synonym of 'chief').
'in charge of' becomes 'ic'.
'what's on' is a charade indicator (letters next to each other).
'top'+'ic'='TOPIC'
Can you help me to learn more?
(Other definitions for topic that I've seen before include "Theme (of, eg, a talk)" , "That may be debatable" , "A subject for conversation or debate" , "might this be hot?" , "Talking point" .)