Bill's about to think, unlike Queen Victoria? (6)
I believe the answer is:
amused
'bill's about to think unlike queen victoria?' is the definition.
I can't tell whether this definition defines the answer.
'bill's about to think' is the wordplay.
'bill' becomes 'ad' (advertisement stuck on wall).
'about' means one lot of letters goes inside another.
'to think' becomes 'muse' (to muse on something is to think about it).
'ad' enclosing 'muse' is 'AMUSED'.
Can you help me to learn more?
(Other definitions for amused that I've seen before include "Queen Victoria wasn't!" , "Mildly diverted or entertained" , "Pleasantly occupied" , "Mildly entertained by Medusa" , "was entertaining" .)