Unpredictable psychopaths are gathering round former drug dealer (11,4)

'former drug dealer' is the definition.
(apothecaries were people who prepared drugs)

'unpredictable psychopaths are gathering round' is the wordplay.
'unpredictable' indicates anagramming the letters.
'gathering' indicates putting letters inside (gather can mean to acquire or bring in).
'round' becomes 'o' (round can mean a round object).
'psychopaths'+'are'='psychopathsare'
'psychopathsare' is an anagram of 'apothecarysshp'.
'apothecarysshp' enclosing 'o' is 'APOTHECARYS SHOP'.