OFFSET constraints (OFFSET/IN and OFFSET/OUT) cover only data paths. These constraints do use the Clock Path to calculate the OFFSET values, but do not actually constrain them. You can constrain these paths with a FROM:TO constraint, but it should not be necessary (especially for clocks on Global Routing).
In case the design requires to have the Clock path constrained, please find below an example to remove the Clock path from the Unconstrained path analysis section.
INST "abc_inst" TPTHRU = abc_grp;
// where abc_inst is the instance name of a MMCM/PLL/DCM/BUFR/BUFIO ...
TIMESPEC ts_ignore_clock = FROM PADS THRU "abc_grp" TIG;