Regular customer's slightly boring tone (7)
I believe the answer is:
habitue
'regular customer's' is the definition.
(I've seen this before)
'slightly boring tone' is the wordplay.
'slightly' becomes 'a bit' (I've seen this before).
'boring' is an insertion indicator (some letters must 'bore' a hole into other letters).
'tone' becomes 'hue' (I've seen this in another clue).
'abit' placed inside 'hue' is 'HABITUE'.
(Other definitions for habitue that I've seen before include "Frequent visitor to a place" , "Regular attendant" , "Regular visitor (to place)" , "Regular customer" , "Person who visits a place frequently, may live there" .)