Amusing duo's opening turn (5)
I believe the answer is:
droll
'amusing' is the definition.
(thesaurus)
'duo's opening turn' is the wordplay.
'opening' says to take the initial letters.
'turn' becomes 'roll' (roll is a kind of turn**).
The initial letter of 'duos' is 'd'.
'd'+'roll'='DROLL'
(Other definitions for droll that I've seen before include "Large amount (of matter)" , "Whimsical - amusing" , "Clownish" , "amusing, in a strange way" , "rum" .)