Period that's quiet in trade (5)
I believe the answer is:
spell
'period' is the definition.
(spell is a kind of period)
'quiet in trade' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano').
'in' is an insertion indicator.
'trade' becomes 'sell' (I've seen this before).
'p' going within 'sell' is 'SPELL'.
'that's' is the link.
(Other definitions for spell that I've seen before include "Give out letters" , "Period (of enchantment?)" , "term" , "Trance" , "Put letters in right order" .)