Form irregular heaps (5)
I believe the answer is:
shape
'form' is the definition.
(synonyms)
'irregular heaps' is the wordplay.
'irregular' indicates an anagram.
'heaps' anagrammed 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" .)