Do one's bit catching swallow (4,2)
I believe the answer is:
beat it
'do one's' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'bit catching swallow' is the wordplay.
'catching' is an insertion indicator.
'swallow' becomes 'eat' (I've seen this before).
'bit' placed around 'eat' is 'BEAT IT'.
Can you help me to learn more?
(Other definitions for beat it that I've seen before include "Get going" , "Scram, get lost" , "Clear off!" , "Sling your hook!" , "Scram, take a powder" .)