Index modulation (IM) has been recognized as a compelling candidate for the next generation wireless networks. This paper considers index modulation-orthogonal frequency division multiplexing (IM-OFDM) with single subcarrier activation, where a single subcarrier is activated for transmitting an amplitude-phase modulated (APM) symbol in each IM group. Furthermore, a reduced-complexity approximate message passing aided symbol detector (AMPD) is proposed for reducing the detection complexity at the receiver side. The system performance of IM-OFDM with single subcarrier activation is investigated in terms of spectral efficiency (SE), peak-to-average power ratio (PAPR), bit error ratio (BER), as well as detection complexity. Our studies show that IM-OFDM with single subcarrier activation is capable of attaining low PAPR. Moreover, IM-OFDM with single subcarrier activation outperforms the classic OFDM using APM for a SE of 1 bit/s/Hz, when high power amplifier (HPA) is employed. Furthermore, our investigation shows that low detection complexity can be achieved by the proposed AMPD for IM-OFDM with single subcarrier activation at the cost of a certain BER performance loss.