A lady of negotiable affection. One who might provide a certain intimate service for reasonable price, and is unlikely to have any use for a needle and thread, unless that's the way the client likes it.
The unwary visitor to Ankh-Morpork is advised that Sandra Battye is also a Seamstress, albeit of a very rare and specialised sort. As sometimes all a man wants is to gain a valuable service from a woman that makes life feel so much better afterwards (such as having the holes in his socks darned against a mushroom), it should be advised here that it is unwise to presume.
For more information see Seamstresses' Guild