Two left, they are interlinked - have ideas for both, but just letters that fit !
52 d) From ocean and lake, an aquatic creature (4) ??A? (Could be CRAB).
51 a) Fashions at the time being decorous (6) M?D???.
The first letter of 52 d)ie C, is the 5th letter of 51 a) - so could be M?D?C?. (Only word I can think of that then fits is Medici. Had thought 51A) could be Modern - but then have no idea for 52 d))