Seven working on key service (8)
I believe the answer is:
evensong
'service' is the definition.
(thesaurus)
'seven working on key' is the wordplay.
'working' indicates an anagram.
'key' becomes 'g' (I've seen this in other clues**).
'seven' is an anagram of 'evens'.
'evens'+'on'+'g'='EVENSONG'
(Other definitions for evensong that I've seen before include "Anglican service" , "6 [SERVICE] ?" , "Church service late in the day" , "Vespers" , "Late afternoon church service" .)