One who may pull out all the stops sang with trio possibly (8)
I believe the answer is:
organist
'one who may pull out all the stops' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'sang with trio possibly' is the wordplay.
'with' means one lot of letters go next to another.
'possibly' indicates anagramming the letters.
'sang'+'trio'='sangtrio'
'sangtrio' is an anagram of 'ORGANIST'.
Can you help me to learn more?
(Other definitions for organist that I've seen before include "Accompanist, keyboard player" , "Roasting (anag)" , "It groans for church musician" , "Church instr. player" , "One plays in church usually" .)