The effect of the primary traffic on a secondary user's (SU) throughput under Rayleigh flat fading channel is investigated. For this case, closed form expressions are derived for the average probability of detection and the average probability of false alarm. Based on these expressions, the average SU throughput under the desired signal-to-noise ratio (SNR) constraint in order to maintain the quality of the secondary link is found analytically considering the random arrival or departure of the primary user. It is shown that the spectrum sensing performance and SU throughput degrade with increase in the primary traffic and the deep fade condition of the channel over which the detection is performed. The degree of degradation in SU throughput is seen to be severed further due to the interference link from the primary transmitter to the secondary receiver. Under these detrimental effects, a sensing-throughput trade-off for SU is illustrated. Finally, the combined effect of the primary traffic, fading, imperfect spectrum sensing and the interference link from a primary transmitter is studied on the outage probability at SU.