Take some pharmacological Mickey Finn and relax (4)
I believe the answer is:
calm
'relax' is the definition.
(thesaurus)
'take some pharmacological mickey finn' is the wordplay.
'take some' indicates a hidden word.
'CALM' is hidden in the letters of 'pharmacological mickey finn'.
'and' acts as a link.
(Other definitions for calm that I've seen before include "Peace, tranquillity" , "Peaceful, undisturbed" , "Serene" , "Halcyon" , "Still and quiet" .)