On reflection it's part of the Kop's ebullient custom (7)
I believe the answer is:
bespoke
'custom' is the definition.
(synonyms)
'on reflection it's part of the kop's ebullient' is the wordplay.
'on reflection' says the letters should be written in reverse.
'it's part of' indicates the answer is hidden within the clue.
'ekopseb' is hidden in the letters of 'the kops ebullient'.
'ekopseb' back-to-front is 'BESPOKE'.
(Other definitions for bespoke that I've seen before include "Description of tailor" , "Made to measure" , "Of clothes, made to order" , "Custom-made" , "Tailor-made" .)