Fools designed shapes to cover poster (8)

I believe the answer is:

sapheads

'fools' is the definition.
('saphead' can be a synonym of 'fool')

'designed shapes to cover poster' is the wordplay.
'designed' indicates anagramming the letters (designed with the letters).
'to cover' indicates putting letters inside.
'poster' becomes 'ad'.
'shapes' is an anagram of 'saphes'.
'saphes' going around 'ad' is 'SAPHEADS'.

I've seen this clue in The Independent.

Want a hint initially instead of a full solution? Install my app