Acknowledges Kelvin in bar (5)
I believe the answer is:
marks
'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?
(Other definitions for marks that I've seen before include "Symbols" , "Visible signs" , "Visible impressions" , "Grades assigned to school work" , "Goals" .)