Simple undergraduate second-in-command (5)

I believe the answer is:

basic

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'simple' is the definition.
(synonyms)

'undergraduate second-in-command' is the wordplay.
'undergraduate second' becomes 'bas' (I can't justify this - if you can you should believe this answer much more).
'in command' becomes 'ic' (military abbreviation).
'bas'+'ic'='BASIC'

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(Other definitions for basic that I've seen before include "At the simplest level" , "Cardinal" , "Straightforward" , "Fundamental, elementary" , "Plain" .)

I've seen this clue in The Mirror.

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