It's popular because the Queen's inside -- that's not true! (9)
I believe the answer is:
insincere
'that's not true' is the definition.
I can't judge whether this defines the answer.
'it's popular because the queen's inside' is the wordplay.
'it's popular' becomes 'in'.
'because' becomes 'since' (I've seen this before).
'the queen' becomes 'er' (Elizabeth Regina).
'inside' means one lot of letters goes inside another.
'since' enclosing 'er' is 'sincere'.
'in'+'sincere'='INSINCERE'
Can you help me to learn more?
(Other definitions for insincere that I've seen before include "Not genuine, feigned" , "may be laying it on" , "Dissembling" , "Not expressing genuine feelings" , "False, hypocritical" .)