Special tonic pacifies condition (13)
I believe the answer is:
specification
'condition' is the definition.
(I've seen this before)
'special tonic pacifies' is the wordplay.
'special' indicates anagramming the letters.
'tonic'+'pacifies'='tonicpacifies'
'tonicpacifies' is an anagram of 'SPECIFICATION'.
(Other definitions for specification that I've seen before include "Detailed requirement of work to be done" , "Detailed description of requirements" , "Condition" , "Requirement, detail" , "Particular detail" .)