Form or class? Not entirely (4)
I believe the answer is:
cast
'form' is the definition.
(I know that cast is a type of solid)
'class? not entirely' is the wordplay.
'class?' becomes 'caste' (type of social class).
'not entirely' means to remove the last letter (most but not all of the word).
'caste' with its final letter taken off is 'CAST'.
'or' acts as a link.
(Other definitions for cast that I've seen before include "'Send forth, a spell perhaps (4)'" , "Allot parts in film, play" , "Discard; actors (in play)" , "'Ne'er .... a clout till May be out'" , "characters together" .)