String trio start on Rioja, for example (5)
I believe the answer is:
twine
'string' is the definition.
('twine' can be a synonym of 'string')
'trio start on rioja for example' is the wordplay.
'start on' says to take the initial letters (I've seen 'starts on' mean this).
'rioja for example' becomes 'wine' (rioja is a kind of wine).
The initial letter of 'trio' is 't'.
't'+'wine'='TWINE'
(Other definitions for twine that I've seen before include "Garden string" , "can get into knots" , "Tough string" , "Coil" , "Stout string" .)