“Art pioneered by Arp”

Lt. ___, communications officer on the original "Star Trek"

Letter between rho and tau

N.H.L. tiebreakers

Allow to attack

Epic that ends "Such was the funeral of Hector, tamer of horses"

Prefix with liberal

With 59-Across, "This is beyond a doubt" ... or a punny hint to 24-, 38- and 49-Across

Online chats, in brief

Nickname for Rihanna