Calm when opening tablets (4)
I believe the answer is:
ease
'calm' is the definition.
(I've seen this before)
'when opening tablets' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'opening' is an insertion indicator (some letters open a gap in others).
'tablets' becomes 'EE' (more than one Ecstasy pill).
'as' placed into 'ee' is 'EASE'.
(Other definitions for ease that I've seen before include "Facilitate; repose" , "No trouble" , "Freedom from worries" , "State of relaxation or rest" , "soldier may be at it" .)