Build up initially appalling service (5)
I believe the answer is:
amass
'build up' is the definition.
(I've seen this before)
'initially appalling service' is the wordplay.
'initially' suggests taking the first letters.
'service' becomes 'mass' (kind of church service).
The initial letter of 'appalling' is 'a'.
'a'+'mass'='AMASS'
(Other definitions for amass that I've seen before include "Put together" , "run up?" , "Get together" , "Accrue, heap up" , "Accumulate, heap up" .)