Shut up about British - essentially, they can't provide a service (7)

'they can't provide a service' is the definition.
(lay people as opposed to service-giving clergy)

'shut up about british essentially' is the wordplay.
'shut' becomes 'seal' (sealing is a kind of shutting**).
'up' says the letters should be written backwards (in down clue: letters go upwards).
'about' is an insertion indicator.
'essentially' means to look at the middle letters.
The central letters of 'british' are 'iti'.
'seal' written backwards gives 'laes'.
'laes' going around 'iti' is 'LAITIES'.