Extra note about ordinary day for groups of swimmers (4)
I believe the answer is:
pods
'groups of swimmers' is the definition.
'pods' can be an answer for 'groups' (I have seen 'Groups of dolphins' mean 'pods' so perhaps 'groups' could also mean 'pods'). I am not sure about the 'of swimmers' bit.
'extra note about ordinary day' is the wordplay.
'extra note' becomes 'PS' (a PS or postscript in a letter).
'about' means one lot of letters goes inside another.
'ordinary' becomes 'O' (abbreviation eg in O-Level, Ordinary Level).
'day' becomes 'd' (abbreviation for day).
'o'+'d'='od'
'ps' placed around 'od' is 'PODS'.
'for' is the link.
Can you help me to learn more?
(Other definitions for pods that I've seen before include "Groups of dolphins" , "schools of whales?" , "From enclosed cabins" , "Seed containers" , "Pea cases" .)