Form irregular heaps (5)
I believe the answer is:
shape
'form' is the definition.
(synonyms)
'irregular heaps' is the wordplay.
'irregular' indicates anagramming the letters.
'heaps' with letters rearranged gives 'SHAPE'.
(Other definitions for shape that I've seen before include "'Figure, outline (5)'" , "Square, circle, etc." , "For good physical condition" , "Heaps (anag.)" , "External form" .)