Friendly agents book in alone (8)
I believe the answer is:
sociable
'friendly' is the definition.
(associated in meaning)
'agents book in alone' is the wordplay.
'agents' becomes 'cia'.
'book' becomes 'b' (abbreviation for book).
'in' indicates putting letters inside.
'alone' becomes 'sole' (synonyms).
'cia'+'b'='ciab'
'ciab' placed into 'sole' is 'SOCIABLE'.
(Other definitions for sociable that I've seen before include "Companionable, friendly" , "Easy in company" , "Fond of mixing with others" , "Marked by friendliness" , "Friendly - gregarious" .)