Fellow isn't commonly vague (5)
I believe the answer is:
faint
'vague' is the definition.
(I know that vague can be written as faint)
'fellow isn't commonly' is the wordplay.
'fellow' becomes 'F' (abbreviation - of a society etc.).
'isn't commonly' becomes 'ain't' (non-standard version of 'isn't').
'f'+'aint'='FAINT'
(Other definitions for faint that I've seen before include "Pale - pass out" , "Swoon - pale" , "Indistinct - pass out" , "dull" , "Swoon, pass out" .)