Rev. is about to lead church worship (7)
I believe the answer is:
service
'worship' is the definition.
(thesaurus)
'rev is about to lead church' is the wordplay.
'about' is an anagram indicator.
'to lead' means one lot of letters go next to another.
'church' becomes 'ce' (abbreviation for Church of England).
'rev'+'is'='revis'
'revis' is an anagram of 'servi'.
'servi'+'ce'='SERVICE'
(Other definitions for service that I've seen before include "See 15" , "Periodic car maintenance" , "Work done by one for the benefit of another" , "start of action in court" , "tree" .)