Check passage that’s blocked by unmentionables, primarily (5)
I believe the answer is:
audit
'check' is the definition.
(an audit is an examination or check)
'passage that's blocked by unmentionables primarily' is the wordplay.
'passage' becomes 'adit' (passage leading to a mine).
'that's blocked by' means one lot of letters goes inside another (block can mean to plug a gap in something).
'primarily' suggests taking the first letters.
The first letter of 'unmentionables' is 'u'.
'adit' going around 'u' is 'AUDIT'.
(Other definitions for audit that I've seen before include "Roman Aquae Sulis" , "Inspection of records" , "Inspection of the account books" , "Examination of accounts" , "Carry out annual check of firm's accounts" .)