Part of bathroom offer best regularly used (5)
I believe the answer is:
bidet
'part of bathroom' is the definition.
(bidets are used in bathrooms)
'offer best regularly used' is the wordplay.
'offer' becomes 'bid' (bid is a kind of offer).
'regularly used' indicates alternate letters (letters selected on a regular basis).
The alternate letters of 'best' are 'et'.
'bid'+'et'='BIDET'
(Other definitions for bidet that I've seen before include "Bathroom feature" , "Washing facility" , "Bathroom fitting" , "Low basin for washing private parts" , "Bathroom fixture" .)