I'm convinced the U & T are correct (see clues / answers below) therefore DOUBT, based on suspicion was my only real thought - although I was trying to work the BUT in some shape or form.
As you have guessed, I was struggling with this one.
Leaning tower perhaps, one protected from being knocked down (6,8)
LISTED / BUILDING
Fit number first that can be defended (7)
TENABLE