Clothes in Crete are not cheap and the choice is limited so it is best to bring what you need with you.
Of course if you have forgotten some hot weather items such as T-shirts, shorts or swimming costumes you will find plenty to buy in tourist shops but don't set your expectations too high.
If you come in spring or in the autumn (especially from October onwards) , remember that it can rain heavily at times and that nights can still be cold so bring adequate clothing.
Luckily, Crete is not the sort of place where you will feel under much pressure to dress-up, especially as a man. You will not stand out if you are well dressed but nobody will mind if you are dressed quite sloppily. Women seem to be (or put themselves) under a little more pressure in this area.
Having said this there are really limits: it seems that a fair number of tourists have no problem going around in pretty "out of place" attire and it is not unusual to see bikini-clad foreigners in the centre of Chania. I doubt that they would go shopping in a supermarket in their own country wearing only a bikini but it seems that once abroad they loose their sense of propriety. This has been going on for too long to shock the locals, they simply think that they are ridiculous.