Each tablet so needed (5)
I believe the answer is:
every
'each' is the definition.
(synonyms)
'tablet so needed' is the wordplay.
'tablet' becomes 'e' (E can mean ecstasy or an ecstasy tablet).
'so' becomes 'very' (I've seen this before).
'needed' says to put letters next to each other.
'e'+'very'='EVERY'
(Other definitions for every that I've seen before include "All, taken separately" , "As much as possible" , "Each of a number" , "Each one without exception" , "Each (one of a set)" .)