Join tuba ensemble (4)
I believe the answer is:
abut
'join' is the definition.
(abut can mean to join or be adjacent to)
'tuba ensemble' is the wordplay.
'ensemble' indicates anagramming the letters.
'tuba' is an anagram of 'ABUT'.
(Other definitions for abut that I've seen before include "Adjoin, press up against" , "Lie next to" , "Lean against the odd tuba" , "Adjoin -- 19 (rev)" , "Of building or land, end or lean against" .)