Son miserable about sweetheart lacking form (9)
I believe the answer is:
shapeless
'lacking form' is the definition.
(I've seen this before)
'son miserable about sweetheart' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'miserable' becomes 'hapless' (synonyms).
'about' indicates putting letters inside.
'sweetheart' becomes 'e' (the heart/centre of 'sweet').
'hapless' placed around 'e' is 'hapeless'.
's'+'hapeless'='SHAPELESS'
(Other definitions for shapeless that I've seen before include "unstructured" , "Lacking definite form" , "unattractive to look at?" , "Amorphous" , "Without definite form" .)