Build up first-rate service (5)
I believe the answer is:
amass
'build up' is the definition.
(I've seen this before)
'first-rate service' is the wordplay.
'first-rate' becomes 'A'.
'service' becomes 'mass' (kind of church service).
'a'+'mass'='AMASS'
(Other definitions for amass that I've seen before include "Assemble" , "Gather in great quantity" , "Stock up on" , "Stockpile" , "store" .)