Sign a chap’s dropped from word book (6)
I believe the answer is:
taurus
'sign' is the definition.
(Taurus is an example)
'a chap's dropped from word book' is the wordplay.
'a chap's' becomes 'hes'.
'dropped from' indicates named letters should be taken away.
'word book' becomes 'thesaurus' (I've seen this before).
'thesaurus' with 'hes' taken out is 'TAURUS'.
(Other definitions for taurus that I've seen before include "A group of stars" , "Zodiacal bull" , "Mountains in 8 across" , "April/May sign" , "house" .)