Varying dose of LSD swallowed by drug transporter (7)
I believe the answer is:
mutable
'varying' is the definition.
(thesaurus)
'dose of lsd swallowed by drug transporter' is the wordplay.
'dose of lsd' becomes 'tab' (tab can mean a square of paper soaked in LSD).
'swallowed by' indicates putting letters inside.
'drug transporter' becomes 'mule' (as in a drug mule).
'tab' placed into 'mule' is 'MUTABLE'.
(Other definitions for mutable that I've seen before include "Inconsistent" , "Changeable" , "It might change!" , "not being fixed" , "it can be changed" .)