It's achievable for one bachelor left in sheriff's group (8)
I believe the answer is:
possible
'it's achievable' is the definition.
(similar in meaning)
'one bachelor left in sheriff's group' is the wordplay.
'one' becomes 'i' (Roman numeral).
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'left' becomes 'l' (common abbreviation).
'in' is an insertion indicator.
'sheriff's group' becomes 'posse' (historical term for a group assisting a sheriff).
'i'+'b'+'l'='ibl'
'ibl' placed inside 'posse' is 'POSSIBLE'.
'for' is the link.
(Other definitions for possible that I've seen before include "Conceivable, practicable" , "Capable of being achieved" , "Conceivable, imaginable" , "Feasible achievable" , "Capable of happening" .)