County surveyor's first book (5)
I believe the answer is:
gloss
'book' is the definition.
(gloss is a kind of book)
'county surveyor's first' is the wordplay.
'county' becomes 'glos' (abbreviation for Gloucestershire).
'surveyor's first' becomes 's' (1st letter of 'surveyor').
'glos'+'s'='GLOSS'
(Other definitions for gloss that I've seen before include "Polish - explanatory comment" , "Superficial appearance" , "Misleading explanation" , "Provide explanations" , "Render plausible" .)