Act of worship (6)
'act of worship' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for service that I've seen before include "mass, say" , "following 28 [SILVER]" , "Act of worship" , "Act of assistance; ceremony" , "Periodic car maintenance" .)
(Other definitions for prayer that I've seen before include "Appeal (to God)" , "Supplication ot God" , "Maybe collect" , "request for help?" , "Orison" .)