The most reserved call for silence? Certainly, it's time! (6)
I believe the answer is:
shyest
'the most reserved' is the definition.
I can't judge whether this definition defines the answer.
'call for silence? certainly it's time' is the wordplay.
'call for silence?' becomes 'sh' (I've seen this in other clues).
'certainly' becomes 'yes' (I've seen this before).
'it's time' becomes 't'.
'sh'+'yes'+'t'='SHYEST'
Can you help me to learn more?
(Other definitions for shyest that I've seen before include "Most diffident" , "Most reticent" , "Most bashful or self-conscious" , "shrinking the most?" , "Most retiring" .)