Fifth generation mobile communication system (5G) integrated with Time Sensitive Networking (TSN) has been deemed as a holistic solution for realizing Industry 4.0, wherein high accuracy of time synchronization is required to enable end-to-end deterministic communication between wireless and wired devices. 3rd Generation Partnership Project(3GPP) TS 23.501 Release 16 has defined the time synchronization mechanism of industrial automation in a 5G-TSN network. This paper briefly reviews the time synchronization scheme of Release 16 and presents the time precision performance considering frequency offset inside 5G System(5GS). It is revealed that frequency offset inside 5GS leads to several microseconds of time error when transmission delay is several tens of milliseconds. As industrial applications require less than 1 microsecond time synchronicity in motion control, the effect of frequency offset inside 5GS needs to be mitigated. In the later part of this paper, a novel method to estimate the frequency offset inside 5GS is presented. The proposed method can be implemented in a simple and effective manner.