Tough Thursday today! And a good challenge, too. I enjoyed it very much, although it took forever to break into. I had a number of scattered answers throughout the grid to begin with, with nary a connection between them (see, that is how you use "nary"), and that was the theme's fault. All these isolated capital letters. What did they mean?
I had my first breakthrough with TROI and ERGO giving me ORALS. Along with LEA and IMAGE in the SE corner, I figured out 50D: Puma, e.g. (SNEAKER). I'd initially thought that, but had rejected it for unclear reasons. I had INTERVAL and IPO in place as well, so the V, I, E, and K gave me INVISIBLEINK. It's a simply lovely answer, and a great revealer which I completely misunderstood for a while. When I got STOOLPIGEON entirely from the pattern of crossing letters, I was stuck looking at the F and wondering what letters had been written in invisible ink. No, really.
I finally figured it out when I muscled FOUNDER into place in 12D. I looked at it, and thought, "well, a clue for FOUNDER would be 'sink'. S[ink]. Ohhhhhhh..." But even once I got the clever theme, it still took me a while because ECCENTRICITY is a tough answer for 18A: K[ink], and MEDIUMRARE is a great left turn for 38A: P[ink].
Partly, also, it's because there are some way out there answers in the NW. TYES is unusual. I wanted "guys", but the U was obviously incorrect. IRIDIC is truly tough for the non-chemistry teachers among us (and I suspect even for them). Who would think that's an adjective for Iridium? That center N section was also hard for me because I put pAl in at 8D: Buddy (MAC). Thus 6A: Depression common during childhood (DIMPLE) took some time. I love the clue, lack of question mark and all.
Much to like here. AEOLIAN is beautiful. IHEARYA and DAREME are nice colloquialisms. HICCUPS is clued very nicely. And the winner for best clue is 14D: Hit back? (REAREND).
Thumbs way up for this one.