What kind of fees can I expect when withdrawing USD to SGD from TD Ameritrade?

The preferential rate with DBS only applies for SGD deposits to their local DBS account. You can call or WhatsApp them to obtain the rate for the day, but it is unlikely that your funds will be converted at that rate because it will only be processed around 2 working days later.

As for withdrawals, TD Ameritrade charges 25 USD. Additionally, because they only allow withdrawals in SGD, your bank may charge you a fee and convert the USD into SGD for you, often at a poor exchange rate.

If you have a US bank account, I would suggest that you to withdraw in USD to your US bank account and use a service like TransferWise to transfer to your SG bank account. Alternatively, you can use a DBS multicurrency account to receive the funds in USD (which will incur a 10 SGD fee) and decide what to do with it next. This can be handy if you want to transfer the USD funds to another broker, and DBS does not charge for USD transfers to bank accounts in the US.