Disreputable son lacking energy to swallow drug (6)
I believe the answer is:
sleazy
'disreputable' is the definition.
(similar in meaning)
'son lacking energy to swallow drug' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'lacking energy' becomes 'lazy' (lazy can mean lacking in energy).
'to swallow' means one lot of letters goes inside another.
'drug' becomes 'e' (E is another name for ecstasy).
'lazy' enclosing 'e' is 'leazy'.
's'+'leazy'='SLEAZY'
(Other definitions for sleazy that I've seen before include "Sordid, squalid" , "Squalid, tawdry" , "Dirty or dishonest" , "Morally degraded" , "Tacky" .)