All sorts of places I set myself back while solving this puzzle. It started with putting "space" in at 5A: Wide keyboard key (ENTER), then Aton for ALOT. Later I took out "space" because I wanted to put dEf in for RES. Following that I misspelled VITTLES with a Z (?!), and then finally stared at the crossing of CGI and GIGS for 15 seconds trying to place the final letter. Better than finishing with an error, I suppose, right?
Oh, and I put MutE in for 36A: Charades player, essentially (MIME). That's a lot of errors to make up for.
Our theme today is a collection of four equine beasts. An ass is a subspecies of equus, while a mule is the offspring of a donkey and a horse, and is famously sterile. All four phrases are relatively strong, with STUBBORNASAMULE and ONETRICKPONY being the best. ONONESHIGHHORSE suffers from the "one's", but is acceptable. THELAWISAASS is the least familiar and the most enjoyable, for certain, but required a number of crosses to become clear to this solver.
BEANTOWN is of course well known to the bloggers on this site. ISIAH returns once again, although not the Isaiah Thomas who was recently traded from Boston to Cleveland, maybe.
1A: Battery fluid (ACID) gets a B-. It's okay.
Fave: SIESTA (42A: Rest of the afternoon). Great clue.
Least fave: ASSN, I guess (1D: The first "A" of N.A.A.C.P.: Abbr.), just because it's an abbreviation from an acronym.
Pretty smooth overall.