what function does the opening part of this scene, with the clown, have?

The minor comic relief that is present is reminiscent of Feste's wit in Twelfth Night. As in Twelfth Night, a man entreats a servant to fetch someone hither; but since the servant is a clown, there is a bit of frustration and word-play before anything is done. The fool-figure receives money, then goes to do his bidding; this is a common set-up for comic relief. However, the clown in Othello does not reappear; he has less personality than Feste, if only because he gets just a brief appearance.

Cassio seems to be one of the only "honest" people in the play; he discloses his plans, his thoughts, and his concerns to Iago, omitting little or nothing. It is therefore ironic that he refers to Iago as "honest", since Iago is the complete opposite of everything Cassio is. By this point,"honest" has become tainted by its over-association with Iago; it has no simple meaning and is a watchword for shady dealings.