This seems to mean something like smoothing out and folding linen, though it would baffle people now.

‘Mary walked to Ambleside for letters, it was a wearisome walk for the snow lay deep upon the Roads & it was beginning to thaw.  I stayed at home & clapped the small linen.’  Monday 21 December 1801

Source: Dorothy Wordsworth, The Grasmere and Alfoxden Journals, ed. and introduction by Pamela Woof (Oxford: Oxford World’s Classics, 2008 (2002)), p. 49