Talk till Tom's out of breath? (4)
I believe the answer is:
chat
'talk till' is the definition.
'chat' can be an answer for 'talk' (chat can mean to talk). I'm not sure about the 'till' bit.
'tom's out of breath?' is the wordplay.
'tom' becomes 'cat' (a tom is a male cat).
'out' indicates putting letters inside.
'of breath?' becomes 'h' (I can't explain this - if you can you should believe this answer much more).
'cat' enclosing 'h' is 'CHAT'.
Can you help me to learn more?
(Other definitions for chat that I've seen before include "Rabbit" , "Talk informally; bird" , "Friendly talk" , "Gossip" , "gas" .)