Wait for one tenor to join in (4)
I believe the answer is:
stay
'wait' is the definition.
(I've seen this before)
'for one tenor to join in' is the wordplay.
'for one' becomes 'say' (both can mean 'for example').
'tenor' becomes 't'.
'to join in' indicates putting letters inside.
'say' enclosing 't' is 'STAY'.
(Other definitions for stay that I've seen before include "Abide" , "Remain; corset part" , "guy to > put off" , "hang about" , "Remain; prop" .)