Basic flat under guard when roof is blown off (5-5)
I believe the answer is:
entry-level
'basic' is the definition.
(eg an entry-level job)
'flat under guard when roof is blown off' is the wordplay.
'flat' becomes 'level' (synonyms).
'under' is a charade indicator (letters next to each other).
'guard' becomes 'sentry' (sentry is a kind of guard).
'when roof is blown off' means to remove the first letter (in a down clue, roof could mean the top letter).
'sentry' with its initial letter taken off is 'entry'.
'level' put after 'entry' is 'ENTRY-LEVEL'.
(Other definitions for entry-level that I've seen before include "Simple enough for a beginner" , "Suitable for beginners" .)