My turn — quiet! (4)
I believe the answer is:
gosh
'my' is the definition.
(both are expressions of surprise)
'turn quiet' is the wordplay.
'turn' becomes 'go' (a person's move in a board game).
'quiet' becomes 'sh' ('sh' is used to tell someone to be quiet).
'go'+'sh'='GOSH'
(Other definitions for gosh that I've seen before include "Wow!" , "Exclamation of surprise" , "Well I'm blowed!" , "Fancy that!" , "Christmas" .)