Behold - behind church - left instrument (5)
I believe the answer is:
cello
'instrument' is the definition.
(cello is a kind of instrument)
'behold behind church left' is the wordplay.
'behold' becomes 'lo' (archaic exclamation meaning 'look!').
'behind' means one lot of letters go next to another.
'church' becomes 'CE' (abbreviation for Church of England).
'left' becomes 'l' (common abbreviation).
'ce'+'l'='cel'
'lo' put after 'cel' is 'CELLO'.
(Other definitions for cello that I've seen before include "Musical instrument like large violin held between knees" , "large-scale fiddle" , "Bass stringed instrument" , "music produced by this?" , "Jacqueline du Pre's instrument" .)