Oftentimes getting a neat head on a fly begins with the very first wrap of tying thread. Don’t start your thread right at the back edge of the hook eye, instead, leave a small amount of space behind it. Let’s call this the “no tie zone”. You don’t want any material or thread wraps in that space until you go to put the head on the fly. This applies to all flies, not just hackled ones like this.
So, the rest of the fly is complete and you’re about done wrapping the hackle. Stop right at the beginning of the “no tie zone” and anchor the hackle there with 2 or 3 tight wraps, no more. Then, go in and snip the excess tip and any stray fibers off as close as possible. The little bit of bare hook shank will allow you to take just a couple of thread wraps to anchor anything that didn’t get snipped. The fewer the wraps, the better.
Finally, finish with a compact 4-5 turn whip finish to secure your tying thread. The result should be a durable head on the fly that’s also small and neat.