Quiet performer enthralling one in performance (8)
I believe the answer is:
taciturn
'quiet' is the definition.
(I've seen this before)
'performer enthralling one in performance' is the wordplay.
'performer' becomes 'act' (someone with an act).
'enthralling' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'performance' becomes 'turn' (turn is a kind of performance).
'act' going around 'i' is 'acit'.
'acit' inserted inside 'turn' is 'TACITURN'.
(Other definitions for taciturn that I've seen before include "Habitually uncommunicative, saying little" , "Reticent" , "Untalkative" , "Aloof" , "withdrawn" .)