We might reckon on these picnic blankets (12)
I believe the answer is:
spreadsheets
'we might reckon on these' is the definition.
(you might do sums or reckon on a spreadsheet)
'picnic blankets' is the wordplay.
'picnic' becomes 'spread' (a spread of food).
'blankets' becomes 'sheets' (as in bed sheets).
'spread'+'sheets'='SPREADSHEETS'
(Other definitions for spreadsheets that I've seen before include "computer programs" , "Organising programs" .)