Join up as part of orchestra (4)
I believe the answer is:
tuba
'part of orchestra' is the definition.
(tuba is a kind of musical instrument)
'join up' is the wordplay.
'join' becomes 'abut' (abut can mean to join or be adjacent to).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'abut' reversed gives 'TUBA'.
'as' acts as a link.
(Other definitions for tuba that I've seen before include "Large low-pitched brass musical instrument" , "item band might play" , "It's not top brass" , "music-maker" , "Large brass wind instrument" .)