Seconds involving extremely rude customs (5)
I believe the answer is:
mores
'customs' is the definition.
(as in social mores)
'seconds involving extremely rude' is the wordplay.
'seconds' becomes 'mos' (mo can mean a short amount of time).
'involving' means one lot of letters goes inside another.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'rude' with its centre removed is 're'.
'mos' going around 're' is 'MORES'.
(Other definitions for mores that I've seen before include "Conventions, manners" , "codes" , "Social customs of a group" , "(Society's) conventions" , "O tempora, O - !" .)