A pitcher has an ERA, and he also has actual stats(hits, xtra basehits, walks, etc).

If his actual stats would produce a 4.0 ERA, then his actual ERA will cause his clutch to be modified.

If his actual ERA was 3.5, then expect clutch to be above a 5. If his actual ERA was 4.5, expect his clutch to be below 5.

The clutch rating is then applied when batters are one base as a modification to the pitcher OPS.

The amount of modification is up for debate, but here is my best guess.

5 is average, no modification.
Each number above or below 5 is a 2% affect on the OPS when runners on.

.700 OPS normal vs righty. Runner on, clutch is 3, thus OPS vs righty is going to be .728

.700 OPS normal vs righty. Runner on, clutch is 7, thus OPS vs righty is going to be .672

Clutch helps make a minor adjustment towards a pitchers actual ERA.