Talk when you see something topping (4)
I believe the answer is:
chat
'talk' is the definition.
(I know that chat is a type of converse)
'you see something topping' is the wordplay.
I cannot really understand how this works, but
'see' could be 'c' (the word for the letter, according to Chambers) and 'c' is present in the answer.
The remaining letters 'hat' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'when' is the link.
Can you help me to learn more?
(Other definitions for chat that I've seen before include "Talk informally; bird" , "Have friendly informal conversation" , "Talk familiarly" , "Small songbird of the thrush family" , "flier" .)