Yes. Both you and your insurer can cancel your policy for third-party accident claim in Singapore. in writing. Upon cancellation, unused portions of your premium will be refunded to you.
Some insurers refund the premium on a pro-rata basis and charge a small administration fee. Others calculate what would have been charged if your policy were a short-term policy, which usually occurs if the cancellation is at your request.
Note that refunds might also be subject to a minimum amount and that your insurer might reserve the right not to refund any premium if a claim has been made.
Please check with your insurer regarding their modus operandi before committing to a policy.