Culminating point of month in service (4)
I believe the answer is:
acme
'culminating point' is the definition.
(I've seen this before)
'month in service' is the wordplay.
'month' becomes 'm' (standard abbreviation).
'in' is an insertion indicator.
'service' becomes 'ace' (ace is a kind of service**).
'm' placed inside 'ace' is 'ACME'.
'of' acts as a link.
(Other definitions for acme that I've seen before include "high peak" , "Summit, crowning point" , "'Highest point, apex (4)'" , "Pinnacle" , "Peak of excellence" .)