Dull sailor is in new hotel (7)
I believe the answer is:
tarnish
'dull' is the definition.
(I've seen this before)
'sailor is in new hotel' is the wordplay.
'sailor' becomes 'tar' (slang term for a sailor).
'in' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'n'+'h'='nh'
'is' going within 'nh' is 'nish'.
'tar'+'nish'='TARNISH'
(Other definitions for tarnish that I've seen before include "Spoil" , "Become dull or blemished" , "Mountain lake" , "Cause to lose lustre, silver or reputation" , "Besmirch" .)