First degree thus fundamental (5)
I believe the answer is:
basic
'fundamental' is the definition.
(similar in meaning)
'first degree thus' is the wordplay.
'first' says to put letters next to each other.
'degree' becomes 'BA' (Bachelor of Arts).
'thus' becomes 'sic' ('thus' in Latin).
'ba'+'sic' is 'BASIC'.
(Other definitions for basic that I've seen before include "Rudimentary - programming language" , "essential" , "Radical" , "'Primary, fundamental (5)'" , "Without frills" .)