Sentence:  ... not One in Fifty, of our Men of Fashion, or of Quality, as they call themselves, has an Understanding that is able to "go alone". They are forc'd, therefore, to wait 'till they have their <WSD>Sustainer</WSD>'s Opinion to lean against, before they dare venture upon the Praise, even of what they are inclinable to think favourably of ... 
Meaning of the word marked with <WSD> tags:  A person or thing that sustains.