The main comings and goings (5)
I believe the answer is:
tides
'comings and goings' is the definition.
(tides come in and go out)
'the main' is the wordplay.
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'main' becomes 'ides' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
't'+'ides'='TIDES'
Can you help me to learn more?
(Other definitions for tides that I've seen before include "Lunar-driven sea movements (5)" , "Periodic rises and falls of the sea level" , "Periodic ebbs and flows of the sea" , "Surges" , "Sea movements caused by moon" .)