Sieve the coffee then dive in (6)
I believe the answer is:
plunge
'in' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'sieve the coffee then dive' is the wordplay.
'sieve the coffee then' becomes 'p' (I am not sure about this - if you are sure you should believe this answer much more).
'dive' becomes 'lunge' (I've seen this before).
'p'+'lunge'='PLUNGE'
Can you help me to learn more?
(Other definitions for plunge that I've seen before include "Thrust into liquid" , "Thrust or dive into" , "Drop steeply" , "'Dive, plummet (6)'" , "Descend steeply" .)