conventional lemon extract is lemon oil extracted via some sort of food-safe solvent (usually a propylene glycol/alcohol combination).
the ingredients on the clubhouse brand pure lemon extract is: propylene glycol, alcohol, water, lemon oil, colour.
i've never used lemon extract myself but safe to assume it would work as well as any glycerine tincture. (which i have done and is fantastic.)
although it would probably be cheaper just to source some propylene glycol and save the lemon extract for aromatising.
hope this helps.