“"It all makes sense now!" … or what you may exclaim after combining the ends of 17-, 23-, 36- and 51-Across?”

Gender-neutral pronoun

Slips a card up one's sleeve, say

Org. that now offers merit badges in Kayaking and Nuclear Science

Mammoth

Portable computer

And so on and so on: Abbr.

Activity that helps one release pent-up feelings

Started, as a new field of interest

"Received and understood," in radio lingo