Dumpy son, unfortunate, without energy (9)
I believe the answer is:
shapeless
'dumpy' is the definition.
(thesaurus)
'son unfortunate without energy' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'unfortunate' becomes 'hapless' (similar in meaning).
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'energy' becomes 'e' (physics symbol as in E=mca).
'hapless' enclosing 'e' is 'hapeless'.
's'+'hapeless'='SHAPELESS'
(Other definitions for shapeless that I've seen before include "Lacking definite form" , "Amorphous" , "unstructured" , "unattractive to look at?" , "Without definite form" .)