It requires time to send rude messages (4)
I believe the answer is:
sext
'it requires time to send rude messages' is the definition.
The definition suggests a singular noun which matches the answer.
'it requires time' is the wordplay.
'it' becomes 'sex' (I can't justify this - if you can you should give a lot more credence to this answer).
'requires' is a charade indicator (letters next to each other).
'time' becomes 't'.
'sex'+'t'='SEXT'
Can you help me to learn more?
(Other definitions for sext that I've seen before include "service" , "Explicit message" .)