28ac is a 'double definition' clue, repent[1] and repent[2] in Chambers.
26a is an abbreviation for 'king' in an abbreviation for stamped addressed envelope (ie 'enclosure wanting something in return'); 'cause' functions as a noun for definition purposes.
Chambers 2016 is even better than 2014 - I've no idea why Azed still recommends the 2014 edition, which had a number of words accidentally omitted that were reinstated in the 2016 update.