Tonight's success was their 19th in a row and 14th out of 14 in Scottish League One.
The home side started brightly as Lee Wallace ran at the Forfar defence, but his shot was blocked.
However, the opening goal came just minutes later. This time Wallace curled in a cross and Clark powered his header into the net.
It was nearly 2-0 inside the first 10 minutes, but Clark's effort was saved after Lewis MacLeod had slipped him in.
Rangers missed a number of chances to double their lead, before Gavin Swankie fired into the top corner to bring the visitors level.
However, Ally McCoist's men raced up the other end and Clark's shot deflected in off Darren Dods.
It was 4-1 just before the break. Ian Black hit the bar with a curling effort and Clark pounced on the rebound for his hat-trick.
Rangers continued where they left off in the second half, creating the majority of the chances and threatening to add to the scoreline.
Clark and MacLeod fired over, but it didn't matter as Clark tapped home from a corner for his fourth of the night.
Dean Shiels nearly got another near the end, but his shot went inches wide of the post.