Paid them back breathlessly! (3)
I believe the answer is:
met
'paid' is the definition.
(I've seen this in other clues)
'them back breathlessly' is the wordplay.
'back' suggests the final letters.
'breathlessly' becomes 'et' (I can't justify this - if you can you should give a lot more credence to this answer).
The last letter of 'them' is 'm'.
'm'+'et'='MET'
Can you help me to learn more?
(Other definitions for met that I've seen before include "Got together" , "Encountered or touched" , "Came across - police in London" , "gathered together" , "Encountered pro tem" .)