Simple soul, thus (5)
I believe the answer is:
basic
'simple' is the definition.
(similar in meaning)
'soul thus' is the wordplay.
'soul' becomes 'ba' (term for the soul in ancient Egypt).
'thus' becomes 'sic' (synonyms).
'ba'+'sic'='BASIC'
(Other definitions for basic that I've seen before include "Forming a foundation" , "Fundamental; alkaline" , "Fundamental -- early computer language" , "Elemental" , "Straightforward" .)