Acknowledges Kelvin in bar (5)
I believe the answer is:
Here is my best explanation:
'acknowledges' is the definition.
The definition and answer can be both related to thinking as well as being verbs in their -s form.
Perhaps you can see a link between them that I don't see?
'kelvin in bar' is the wordplay.
'kelvin' becomes 'k' ('K' can be a synonym of 'kelvin').
'in' is an insertion indicator.
'bar' becomes 'mars' (I can't justify this - if you can you should believe this answer much more).
'k' going within 'mars' is 'MARKS'.
Can you help me to learn more?