Book cancelled by popular scientist (6)
I believe the answer is:
boffin
'scientist' is the definition.
(boffin is a kind of scientist)
'book cancelled by popular' is the wordplay.
'book' becomes 'b'.
'cancelled' becomes 'off' ('off' can be similar in meaning to 'cancelled').
'by' says to put letters next to each other.
'popular' becomes 'in'.
'b'+'off'+'in'='BOFFIN'
(Other definitions for boffin that I've seen before include "Scientist (in the back room?)" , "Back-room boy" , "Technical expert" , "Research scientist" , "Technical researcher" .)