Settle for drop in customer numbers (8)
I believe the answer is:
footfall
'customer numbers' is the definition.
'settle for drop' is the wordplay.
'settle' becomes 'foot' (in the sense of footing a bill).
'for' says to put letters next to each other.
'drop' becomes 'fall' ('fall' can be a synonym of 'drop'**).
'foot'+'fall'='FOOTFALL'
'in' is the link.
(Other definitions for footfall that I've seen before include "Sound of someone walking" , "Sound of step" , "Customers" , "Commercial patronage" .)