Fellow lied about discipline (5)
I believe the answer is:
field
'discipline' is the definition.
(I know that discipline can be written as field)
'fellow lied about' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'about' indicates an anagram.
'lied' with letters rearranged gives 'ield'.
'f'+'ield'='FIELD'
(Other definitions for field that I've seen before include "Branch of study" , "Piece of grassland" , "Pasture or crop land" , "Area of farmland" , "Area of interest" .)