Let's pray, playing an old instrument (8)
I believe the answer is:
psaltery
'an old instrument' is the definition.
(ancient string instrument)
'let's pray playing' is the wordplay.
'playing' is an anagram indicator.
'lets'+'pray'='letspray'
'letspray' is an anagram of 'PSALTERY'.
(Other definitions for psaltery that I've seen before include "plucky thing" , "Dulcimer-like instrument" , "Plucked instrument; play rest (anag.)" , "string instrument of yore" , "something demanding pluck" .)