Number of customers in time to pay for trip (8)
I believe the answer is:
footfall
'number of customers in time' is the definition.
(number of visitors to a shop in a certain period)
'pay for trip' is the wordplay.
'pay for' becomes 'foot' (as in to foot the bill).
'trip' becomes 'fall' (fall is a kind of trip).
'foot'+'fall'='FOOTFALL'
'to' acts as a link.
(Other definitions for footfall that I've seen before include "Sound of step" , "Customers" , "Sound of someone walking" , "Commercial patronage" .)