Upbeat lines written by ambassador in church (6)
I believe the answer is:
cheery
'upbeat' is the definition.
(thesaurus)
'lines written by ambassador in church' is the wordplay.
'lines' becomes 'ry' (abbreviation for railway).
'written by' means one lot of letters go next to another.
'ambassador' becomes 'he' (His or Her Excellency).
'in' means one lot of letters goes inside another.
'church' becomes 'ce' (abbreviation for Church of England).
'he' placed within 'ce' is 'chee'.
'ry' put after 'chee' is 'CHEERY'.
(Other definitions for cheery that I've seen before include "In good spirits" , "Merry, jovial" , "Joyful -- optimistic" , "Bright - pleasant - good-humoured" , "Happy, optimistic" .)