Conversation in pub about radius in diagram (3,5)
I believe the answer is:
bar chart
'diagram' is the definition.
'conversation in pub about radius' is the wordplay.
I cannot quite see how this works, but
'pub' could be 'bar' (both are drinking establishments) and 'bar' is present in the answer.
'about' could be 'c' (abbreviation for 'circa') and 'c' is found in the answer.
'radius' could be 'r' (abbreviation in geometry) and 'r' is located in the answer.
The remaining letters 'hat' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'in' acts as a link.
(Other definitions for bar chart that I've seen before include "Diagram showing figures" , "graph" , "Table" , "display of information" .)