Most retiring from business having OK time (6)
I believe the answer is:
coyest
'most retiring' is the definition.
(coy can mean retiring or shy)
'business having ok time' is the wordplay.
'business' becomes 'co' (short for company).
'having' means one lot of letters go next to another.
'ok' becomes 'yes' (OK can mean yes).
'time' becomes 't' (abbreviation).
'co'+'yes'+'t'='COYEST'
'from' acts as a link.
(Other definitions for coyest that I've seen before include "Most modest" , "Most reluctant" .)