When it's only tiny, keep quiet about a bit of pain (6)
I believe the answer is:
minute
'when it's only tiny keep' is the definition.
I can't judge whether this defines the answer.
'quiet about a bit of pain' is the wordplay.
'quiet' becomes 'mute' (mute means quiet or silent).
'about' means one lot of letters goes inside another.
'a bit of pain' becomes 'in' (I can't justify this - if you can you should give a lot more credence to this answer).
'mute' going around 'in' is 'MINUTE'.
Can you help me to learn more?
(Other definitions for minute that I've seen before include "Really very small, like a period of time" , "Little -- 60 18 across" , "60 seconds; tiny" , "Small; short time" , "Immeasurably small" .)