Simple angle concealing facilities getting erected (7)
I believe the answer is:
'simple' is the definition.
(simple can mean foolish or stupid)
'angle concealing facilities getting erected' is the wordplay.
'angle' becomes 'fish' (synonyms).
'concealing' means one lot of letters goes inside another.
'facilities' becomes 'loo' (I've seen this in other clues).
'getting erected' shows that the letters should be reversed in order (in a down clue, letters go up).
'loo' back-to-front is 'ool'.
'fish' placed around 'ool' is 'FOOLISH'.
(Other definitions for foolish that I've seen before include "Silly, lacking common sense" , "Not very bright" , "Idiotic" , "Silly, unwise" , "Absurd, not sensible" .)