Calmer, therefore different (7)
I believe the answer is:
soother
'calmer' is the definition.
(I've seen this before)
'therefore different' is the wordplay.
'therefore' becomes 'so'.
'different' becomes 'other' (similar in meaning).
'so'+'other'='SOOTHER'
(Other definitions for soother that I've seen before include "form of painkiller?" , "pacifier" , "One with calming effect" , "calmer" .)