Talk then quietly catch one's breath (4)
I believe the answer is:
gasp
'catch one's breath' is the definition.
(I've seen this before)
'talk then quietly' is the wordplay.
'talk' becomes 'gas' (I've seen this before).
'then' says to put letters next to each other.
'quietly' becomes 'p' (p for piano and quiet).
'gas'+'p'='GASP'
(Other definitions for gasp that I've seen before include "Catch one's breath in astonishment" , "Puff, struggle for breath" , "First Greek letter" , "Gaps in struggle for breath" , "Convulsive breath" .)